How so?
It means they're playing the same sport in the same way generating the same number of incidents but those incidents are more serious potentially due to one change in variables - the material used to make the pitch.
It is, of course, still possible that it's due to another factor but since it's a statistically significant finding at this stage, and taken over five years, they've decided to investigate.
I find the fact that the surface has the potential to exacerbate injuries which occur, but not to be a factor in causing any which otherwise would not have occurred, to be very strange.
Hamstrings are one listed, for example. It could be that the faster surface factors into hamstring tears because people are spending more time sprinting at high speed, but why would that only affect the severity and not the number?