You'll find that in the JEDI novelization it was actually Ree-Yees and Ephant Mon who have this argument. The Ephant Mon puppet even had a little hole in the shoulder to have Salacious B. Crumb sitting there. Either way, the scene was shot with Yak Face and Ree-Yees with Salacious B. Crumb kibitzing on the side.

In the novelization, the argument ends up with Ree-Yees pounding Ephant Mon in the snout. It appears that in the film version (which was deleted in the end) C-3PO is the one who gets knocked over somehow. It probably made for more of a laugh.

The pictures below illustrate the order of events. Yak Face is sitting down at first while C-3PO appears to be translating. Then you will see poor 3PO knocked down onto the couch behind him by Yak Face. Here's the passage from the novelization:

Threepio was way out of his depth. At the moment, he was being forced to translate an argument between Ephant Mon and Ree-Yees, concerning a point of quark warfare that was marginally beyond him. EPhant Mon, a bulky upright pachydermoid with an ugly, betusked snout, was taking (to Threepio's way of thinking) an untenable position. However, on his shoulder sat Salacious Crumb, the insane little reptilian monkey who had the habit of repeating verbatim everything Ephant said, thereby effectively doubling the weight of Ephant's argument.

Ephant concluded the oration with a typically bellicose avowel. "Woossie jawamba boog!"

To which Salacious nodded, then added, "Woossie jawamba boog!"

Threepio didn't really want to translate this to Ree-Yees, the three-eyed goat-face who was already drunk as a spicer, but he did.

All three eyes dilated in fury. "Backawa! Backawa!" Without further preamble, he punched Ephant Mon in the snout, sending him flying into a school of Squid Heads.

See Threepio felt this response needed no translation., and took the opportunity to slip to the rear - where he promptly bumped into a small droid serving drinks. The drinks spilled everywhere.