Good point. The average NFL career is short and there are lots of injuries every year. If player has a long career, the injuries can even out (see Willie McGinest). If a player is less talented and suffers one or more serious injuries, he seems "injury prone."
There are some players who can't play with injuries or are indeed prone to injury (Wheatley had something wrong with his wrists, others have seemed to not have a solid enough frame or congenital disorders).
Mostly, though, it's just a couple bad rolls of the dice IMO.
To me, injury prone means "more susceptible to wear-and-tear" injuries or "less likely to play through wear-and-tear injuries", or both. If a guy tears his ACL, that doesn't make him injury prone (maybe if he keeps re-tearing the same ACL, to the point that you have to figure it's compromised in a way that makes him more a lot more prone to re-injury). If he's consistently sitting out with stuff like tendinitis, turf toe, hamstring issues, groin pulls, back problems, etc., then he's probably injury prone.
The key, in this case, is that these are the sorts of injuries that some guys just seem to be more prone to, and it's also the kind of injury that some guys will play through (and remain effective) and others won't. There's also a pretty large element of hindsight/confirmation bias. Once you've decided that a guy is injury prone, it's easy to look back through his history and cherrypick all of the times that he's showed up on the injury report, and the resulting account will make it look like he's always injured. What gets lost in that is that you can do that for almost every player. Even the iron men are showing up on the injury report all the time. And draft hindsight is even worse. For every Ras-I Dowling, who drops due to health concerns and those issues carry over into the pros, there's a Chandler Jones, who dropped for the same reason but has only had one injury--of moderate significance, which he played through--in two seasons of heavy workload.
The really hard ones to classify are guys who seem to consistently suffer "freak" injuries - the sort that could happen to anyone with one unlucky hit. Yet, in a lot of cases, these injuries seem to cluster on certain individuals. Jon Beason is a great example of this. Most of his injuries have been of this type: torn ACLs, blown achilles, messed up shoulder, etc.; but just considering how many of these injuries he's had, it's impossible not to conclude that he's injury prone. Gronk is bordering on this territory as well.
