Answer:
Because they don't use a variety of muscles as much as they could by doing more sports so when they use the lesser used muscles, they could get hurt.
Explanation:
Heavy weight lifting is fitted on Anaerobic Activities
Heavy Weight lifting is considered short lasting-high intensity training which relies on on muscle strength rather than cardio vascular capabilities ( if it relies on cardio, it fitted on Aerobic Activities)
other Examples of anaerobic activity : Pull ups, Bench Press , Push up, etc