Disturbing the peace, mainly, by causing a stink and bringing attention to yourself, or trespassing in the secure area without permission or clearance. Also, if the LEO backed up TSA and told you to complete screening and you walked away, then you could also be arrested for not following a lawful order from a peace officer (whether it was lawful or not in the first place wouldn't get decided until you went to court).

If you did have contraband, the LEO could also arrest you for that, as well, provided you violated local laws, which is usually the case with locally owned airports.

Basically, a cop can arrest you for any reason he wants, and if he's sympathetic to TSA (or just scared of them), he'll make up a reason and then let the judge sort it out. It's all about intimidation, both with TSA and the cops.