I'm not sure, it may have been a situation that slowly developed over time. Off the top of my head, a lot of his early injuries involved contact. He fractured his clavicle when a DB landed on him during a tackle, he broke his ribs when Savage threw him into a hit, and his ACL happened during a tackle that was nearly a horse collar. So a lot of those games missed were not due to muscular injuries.
From what I've read and heard about Fuller I think there are 2 issues that have hurt him. First, he had a rash of muscle injuries in the season after his ACL. Its important to remember, rehab isn't the same as training. You have to give injured/repaired tissue time to heal, once they have healed properly you can begin training. When you can run a sub 4.3 forty you can put tremendous forces on the body, he may not have had the necessary time to allow his body to recover from the acl and get his body ready to handle the rigors of the season.
Second, and this is speculative, there is a concept in training called force/velocity profiling. You basically analyze the 10yd splits of a 40yd dash and you can determine if the athlete is force dominant (think of a big strong powerful WR like Andre Johnson) or velocity dominant (think of smaller guys like Fuller or maybe Deshaun Jackson). This is useful information because it can give you a pathway to improvement for the athletes. If you show force dominance you need more velocity training. If you are velocity dominant you need more force/strength work. My guess with Fuller is that he is velocity dominant, and I'm basing this off his combine numbers - sub 4.3 forty but he only jumped 33" in the vertical jump. The VJ has a large force component. There was some early research that seemed to indicate that velocity dominant athletes were prone to hamstring issues. This was discussed at a seminar I attended in '16 or '17. I'm not sure what's come of that, if they definitely proven this or not. It's an interesting concept though and could apply to Fuller, especially if you combine the two scenarios, coming off an ACL and being a velocity dominant athlete.
BTW, Really like the new guy they hired Matt Van Dyke. He's a smart guy. I've followed his work for a while