BobTDQ tried to betray Carolyn before BRob betrayed him.

I think it got a bit lost because it was at the end of episode 3...but...

BobTDQ and Danielle wanted to force Carolyn into the coffin to throw her under the bus. BobTDQ said, verbatim "She don't have a choice. If we say she's in, she has to." BRob argued that putting a Traitor in the coffin would expose them and Bob kept pushing for it. Danielle said "Let me try to convince her". BRob actually saved Carolyn in that argument by refusing to form a majority.

So BRob had already clocked that BobTDQ was gunning to throw a fellow Traitor to the wolves BEFORE BobTDQ said the whole "one of the three new guys is a Traitor" thing.