And for some reason ( at least based on what I've read ) it seems to occur on the right side more often then on the left side - I don't know why the right side is more susceptible. There a a couple of theories that have tried to explain why ETAP occurs but there is no one clear cut explanation as to why ETAP occurs - at least as far as I know.
That said, the reason I most often see for ETAP is improper hydration. So, some form of dehydration may be part of the explanation. I've also heard that if you eat a meal too soon prior to running / exercise, it may also bump the likelihood of ETAP.