Sincilbanks
Vital Football Hero
I think that is a big factor, fatigue is one thing, but having some 14 stone centre half smashing into your leg or your ribs takes it toll. For every challenge that actually causes an injury there and then on the pitch, I'd say there are 3 or 4 in each game that may hurt at the time but won't really hurt until a few hours later and then will be aching and sore the next day.
Not all games are like that ofcourse, but plenty of them will be especially when playing against those teams at the bottom who will be scrapping to get points at any cost.
It's not just those more obvious ones though, the tussles in the 18 yard box for a loose ball, the set pieces where many players in a small area going for the same ball, all of those come with some element of physical contact and eventually you get an elbow in the ribs or a light-ish kick in the calf or some such and over the course of many games in a short space of time they build up.