Before he became a media mogul, Bill Simmons was a Celtics fan. While Simmons was getting super famous, the Celtics weren't very good. The man overseeing that not-very-goodnesss was coach Doc Rivers. So Bill's problems became Doc's problems, and vice versa.
This is what happens when one of the biggest stars in sports media cares deeply and publicly about a particular team, and the guy running team isn't just going to take it all in stride. It got awkward, and it got beefy.