Yeah, I read that it's a part of his routine and it's one of the parts of his job he enjoys doing. From what I read on the Yankees site, he was fielding one out there and he lost his footing and knee buckled. That hurts just thinking about it.
There is video of it and it looks to me like he was putting "too much" effort into catching a fly ball by the wall during batting practice. Easy to say that afterwards, but I bet they will think twice about what they let the players do when shagging balls from now on.