The show implies that it’s because of his sex tape with Jules , who’s a minor. So basically statutory rape/sex with a minor.

But I have a theory: I feel that Nate called the cops on cal because of that one drunk night where he left the house. Nate couldn’t stand to see his dad happy , so maybe he said it was a domestic abuse issue or something like that. I think he did this because he wants to “own” his had and fuck with him psychologically.

I don’t think he ever gave the sex tape to the cops because he still has the flash drive. The cops would’ve immediately confiscated the tape as evidence against Cal. It’s also important to realize the cop never states why Cal is being arrested , so it’s gotta mean something that they left that out.