They don’t call him a pimp for nothing.
Here’s how TMZ reports it:
“There’s no audio, so we can’t tell what the argument is about, but by the looks of the employee’s body language before the wallop (placing his hand on his chest) the guy wasn’t looking for a fight. Then out of nowhere Katt winds up and slaps him in the face—at which point the employee pulls out his phone and, according to an eye witness, he calls police.”
See the video below of the slap, followed by another video of the aftermath: