This is from a 1994 episode of The Tonight Show host by Jay Leno. This clip begins after Leno interviewed Burt Reynolds, who is sitting to the left of an empty chair reserved for the next guest, Marc Summers (host of Nickelodeon's game show Double Dare). Before Summers comes out, Leno shows a scene from Double Dare where Summers gets repeatedly hit by pies from some kind of pie throwing machine.
Leno asks him about it, and Summers says that it is weird for him to be the host of a messy show because he is a "neatness freak." (Summers has OCD) For some reason, that strikes a nerve with Reynolds, who starts making sourfaced verbal jabs. Summers makes a crack about still being married (Reynolds had just gone through a messy divorce with Loni Anderson) and Reynolds grabs a mug of water and tosses it on Summers. It goes downhill from there. Leno does nothing to stop it because he knows good TV when he sees it.
If you want to know the backstory (and it's fascinating to hear Summers describe it), here's the chapter about the incident from Summers' book, Everything In its Place: My Trials and Triumphs with Obsessive Compulsive Disorder.