This one was filed by five protesters who allege they were roughed up by Trump's security detail outside Trump Tower when he was running for President. Considering there's video footage of the alleged assault and at least one of them ended up in the hospital, they look like they have a good case.
Trump himself was sucked into the lawsuit more than just as the security detail's employer because of statements he made that sure sound like he was condoning the assaults. Trump said in recorded comments that the injured protester "should have been roughed up because it was absolutely disgusting what he was doing."
This is not the first time Trump has been sued for supporting violence against protesters. In this one, Trump is fighting through his lawyers the whole idea of him appearing and being forced to testify, even though state law requires it. Hopefully this matter will continue to trial so that he will learn there are consequences for his actions.
While the Supreme Court has held that a sitting President may be compelled to testify in a federal civil suit, whether or not this theory extends to state courts has never been tested. It would be delightful for this point to be fully litigated and then for Trump to lose.
He seems to think he's above the law. When he's ruled against by a judge, it's always a good day because he's reminded, once again, that no one is above the law, not even a President.