First off, unfortunately I've never heard the actual broadcast. This was pre-Internet/pre-SportsCenter, so I've only been able to get a few bits and pieces of it. But here is independent confirmation from a 1977 SI story that it did happen:
http://sportsillustrated.cnn.com/vau...2297/index.htm
The background to this is that Red Auerbach was a vindictive, shriveled cuss who hated the ABA and hated the merger. He made sure that, unlike with the AFL-NFL merger, ABA player records would never be combined with NBA records, even though the first year of the merger proved that the talent difference between the leagues was nonexistant.
Johnny Most was an overrated POS announcer who was only known because the Celtics were winning championships (think of how Sam Smith got a reputation by being the Bulls beat writer; once the Bulls stopped winning, he was exposed as being pretty much a hack). Most's broadcasts were
horrible. I know whereof I speak because unfortunately I heard many more of Most's broadcasts than I did of Stembridge's. Celtic fans loved him in the same way one loves a crazy demented uncle who farts and drools on himself while his friends look on in horror. Unlike Chick Hearn, who was truly a great broadcaster, Most was a complete homerific joke who related none of what was actually going on in the game. (Basically Tommy Heinsohn's entire shtick is watered-down Most.)
Anyway, the details are sketchy, but the genesis of the fight undoubtedly had to do with Most making some Auerbach-fueled disparaging remarks on the air about the ABA, and Stembridge took umbrage. No actual punches were thrown, but apparently it got pretty close.
(Interesting side note in trying to Google some stuff: apparently Terry Stembridge, Jr. was the Vikings' PBP announcer for the 2001 season--when Red McCombs owned the team--but he did not do too well.)