You are just breathing wrong dummy. (Um, maybe not. It is a little more complicated than that.)

There is a paradoxical idea that the fitter someone is, the more likely they will experience respiratory limitations.  This referenced article today suggests that every endurance athlete "has their own limit in endurance training", and that once exceeded it will produce all the possible respiratory disorders discussed in the article.

This article suggests there is a debate in the scientific community as to whether the lung can be defined “overbuilt” or “underbuilt” for facing strenuous exercise. In the ideal scenario,  your athlete will have a respiratory system perfectly tailored to meet their body’s metabolic demands under normal conditions.  However, it is when challenged by demand, whether that be intensity of exertion, environmental challenges or underlying physical pathology where limitations can impact the athletes demand and performance, thus, a pathological response can occur in a seemingly healthy athlete.  
Breathing, how to do it right, how you are doing it wrong. It is all over the internet these days and there are so many "experts" teaching it now. Some are also teaching it and diaphragm "activation" as the answer to every ailment you have, including why you received a "B minus" grade on your 6th grade spelling test. 
Make no mistake, how to breath properly is important. But, like much of the work Ivo and I do, and much of the preaching we do here on The Gait Guys is about getting to the root of the problem. Converting someone to diaphragmatic breathing from a thoracic cage breathing pattern (use whatever nomenclature you wish, we are trying to keep it simple here) is important, but not as important as finding out why someone is doing it. So are you looking deep enough? Are you asking the right questions before you just assume they forgot how to abdominally breathe ?  All to often we have our clients and athletes come in with their newest epiphany from their latest alternative "guru". Lately is it is, "my yoga/pilates/trainer/coach/bodyworker/massage therapist etc showed me how to belly breathe and use my diaphragm properly ! All is going to be ok now ! Everything will now be right in the world !"  However, all to often they fail to realize that all of the things this article delves into, not including the obvious things such as posture, thoracic spine mobility and stability, prehension patterns, workout habits, soft tissue tension/shortness/tightness, muscle weakness, motor pattern aberrancies, etc these are all just a piece of the potential "causes" of the breathing choice and problems. Fixing the problem helps to allow the natural breathing pattern to occur, with some helpful correction and re-education of course. 

There is a paradoxical idea that the fitter someone is, the more likely they will experience respiratory limitations. One's respiratory abilities as an athlete must be built up, just like any other component of their training. The lungs must be trained to satisfy the metabolic demands of the system, however, often their are parameters existing that are outside of the athletes training efforts. Ask any athlete who jumps into altitude training and this becomes painfully apparent. Endurance athletes do die, thankfully not very often, but they do die and it is not always directly from sudden cardiac failure. There is often a reason the endurance athlete dies shortly after the event, not during, when the physical exertion was actually occurring. Think about it. Exercise-induced respiratory disorders do exist in athletes and they are often the limiting factor in excelling physically. Ask any runner who has done a cold weather run, they will often be able to describe the thermal stress of cold air inhalation and dehydration. This is airway cooling followed by rewarming cycle. The coldness triggers a parasympatheic driven brochoconstiction and a vasoconstriction of the brochial venules. Subsequent rewarming follows and the opposite happens, followed by mucosal edema. 

This article proposes: "the question is precisely to understand if there is a limit in terms of intensity and/or in terms of duration in years to endurance training, before respiratory disorders can appear, and if we can apply any preventive strategies. To be an endurance champion, this inevitably means accepting all the labors of strong training but also enduring all possible health problems caused by the same."

There are many factors to consider, asthma, exercise induced asthma, temperature intolerance to cold or warm weather, a sensitive bronchial tree, long time smoker, prior smoker, medications, bronchospasms, reduced ventilation abilities, pulmonary edema, allergies . . .  the list goes on an on, read the article.

One must consider all of those cardio-respiratory limiting pathologies, but, do not forget posture, faulty breathing technique, tight scalenes and pectorals, weak abdominals, poor thoracic rotation and extension, faulty arm swing, protracted shoulders and the list goes on. And, even more so, think of all the things we do when we started getting "winded" as an athlete, we move into more chest wall breathing, tap into the accessory respiratory muscles and all the pathologic patterns that go with it. We begin to struggle, first subtly, then more profoundly until we must stop. Now, do that several times a week and see what happens to your breathing habits. Respiration in the sedentary and in the athlete is a real issue, but it is multifactorial and complex.  It is more than, "Mr. Jones, please lie down. I am now going to teach you to abdominal breath and use your diaphragm correctly (because I just went to a seminar) and all of the stars will align and your next born child will become the next Michael Jordan."  Don't be that guy/gal. 

Breath deep my friends.

Have a read of the referenced article , it should open up your world as to how complex this machine truly is.

-Dr. Shawn Allen, the other gait guy

Respiratory disorders in endurance athletes – how much do they really have to endure?

Maurizio Bussotti, Silvia Di Marco, and Giovanni Marchese
Open Access J Sports Med. 2014; 5: 47–63.
Published online 2014 Apr 2. doi:  10.2147/OAJSM.S57828