This problem expands past European Football, too. I've heard of several local high school boys stroking out on the Football (american) practice field lately and a couple of band students.
I had an old coach explain it to me as best he could and he says alot of it has to do with conditioning. When he was playing football in the 50s, they didn't get water breaks or anything, but they also didn't have stuff like air conditioning and gatorade to help hydrate them growing up. With the introduction of those and other innovations, certain conditions need to be made to ensure these athletes' safety.