What do you think causes all these pre-season injuries?

The sheer amount of injuries suffered by NFL players before the season even begins, whether that's in training camp or mandatory/voluntary workouts beforehand, is startling and unprecedented. You don't see this at other levels of football. Practice injuries happen at other levels, but not at that rate. With the rash of injuries we see every summer during the lead up  to the NFL season, what is going on that causes these injuries?
 
In my opinion, it's all the drills that work the joints so heavily. Franchises work very hard on footwork and technique, and while that is valuable, I do wonder if they need to ease the pressure they're putting on players' joints during practice.  You also have to factor in that it's hard to maintain so much surface and there are sure to be questionable areas.
 
Yeah, reps matter, but so does a guy's availability come September.
 
What are your thoughts?

There's far more contact in football than in any other sport.
 
The NFL also has the longest offseason of the four major sports.  Some guys just get out of shape over the course of four or five months and come into camp thinking they're ready for the conditioning drills.

Millenials are just weak