Helpful video: https://www.youtube.com/watch?v=Kyc_ysVgBMs

But basically, the picture was a cropped frame from a CSAM content, which then their systems thought you are posting CSAM content when you did not.

About the legal consequences, i am not a lawyer, but i don't think you will be visited by the police anytime soon, since the picture you posted isn't CSAM by itself, just a cropped portion which does not contain the material itself.

Edit: as someone said, it goes through multiple human reviews.