Maybe he knew the trap, but he wanted to continue to play instead of winning after few moves
Alexander Alekhine misses Basic Opening Trap That Wins a Rook!

Was he bored and wanted the game to be more interesting? He was saving Qf3 as a secret weapon, in case Gadjukevic opposed him in a more important setting later?

That's funny, I have a Petrosian game where he overlooked a basic tactic that would have given his opponent a clear advantage:
I was annotating games as a practice months ago and then ran it past a computer afterward.
Former world champion Alexander Alekhine played a game against Gajdukevic (a chess player) and missed a well known beginner's trap in the Queen's gambit accepted. Why did this happen?