Too much, too soon, with too little preparation........
Hi repetitions are a hallmark of gymnastics training, and are an absoluted must too burn skills into mental and muscle memory. The problems come about when coaches see work with kids who's physical condition lags behind their learning and training rate.
Use the past olympic teams as an example of what physical preparation looks like as a final product, and ask yourself why these kids are so muscular. The simple answer is that strength combined with technique makes the sport easier, and the difficult skills easier to do. More than that.......These peak levels of fitness and a respect for the stress these skills place on our bodies reduces the chance (eliminate...I wish) for hi rep/overuse injuries.
The next questions to pose are has the gymnast been physically trained to support the stresses of the skill, and are these stresses being spread over an adequate period of time. Additionally, do the coaches recognise that one good attempt, or even a handfull of them, do not indicate the kid is ready for the "next step". I cringe when watching a coach "air spot" a kid on a skill, and proclaim..."you've got it" and then expects the kid to work the skill like a seasoned veteran. The reality, more often than not, is that the kid is making progress towards a destination but hasn't really arrived, and won't for a while.