It is not nessesarily something to worry about.
First off how old are the girls? Young kids aged 10 and under tend to be quite resilient o injuries, they bounce around all the time and fall down a lot and seem to just pick themselves up and carry on a lot. But kids going through the puberty stage (around 11-14) are often prone to injuries. At this age their bodies are growing and changing rapidly, leaving the bones, muscles and tendons more vulnerable to injury. This is a normal part of their development.
Secondly optionals is a big step up from compulsory. The hours are longer, the training sessions and more intense and the skills are a lot bigger. Some kids cope with the higher training intensity with ease, others do not. Everyone's body is unique. Some body's can handle 30 or more hours of training with ease, while other bodies might be exhausted on 10. Those bodies will be more prone to injury. And of course with higher skill levels, little mishaps do happen.
Thirdly, gymnasts tend to be active kids in general. Especially those who make it to optionals. Active kids often get hurt in other ways - playing sport at school, falling of hover boards, climbing trees, jumping off the roof into their swimming pools, trying crazy tricks at home on their trampoline, showing off their gymnastics to their friends on the concrete and so on.
It could be purely coincidental that a third are injured. One way to see if it is related to the gym or not is to look at how the gym deals with injuries. Do they encourage a visit to the physio, do they allow training to be modified or do they expect the kid to continue to push through the pain?
