I think it's a consequence of the increased professionalisation we've seen in the last 10-15 years. The players are far better athletes than they used to, much better conditioned but also as a result they push their bodies to the max to a far greater degree than previous footballing generations did. Unfortunately that can lead to some bodies and hearts snapping if a potential condition isn't discovered in time.
I remember hearing about this the other day and seeing him collapse. So sad. I remember watching this kid in the Euro u17 final in 2014 when Onomah played. He had a really good game. I have never forgotten his name as I remember him getting an assist with one of the most ridiculous passes I've seen in a match. Fairly confident it was him, his back was to goal, and the ball was rolling towards him and he stabbed his foot forward under the ball, the ball top spinned of him and the defender into the path of the attacker. I will try and find it again. Pretty sure it was the final, I know they played twice.
EDIT - wasn't him but I do remember him having a good game. Goal here