Howard: Korchnoi was in severe time-trouble from moves 37-40.
More specifically, Spassky's 37.Rc3 was a mistake, as CL&R pointed out back in the day, for it allowed Korchnoi to play 37...Qd8 forking both of Spassky's rook pawns.
But....Spassky was also in time trouble, which is why he replied (Get this!) 38.g3??. This move not only doesn't protect the KRP, but it makes 38...Qxh4 even stronger than it otherwise would have been!
But, alas, Korchnoi was blitzing his moves too fast, and that's why his queen went for the other unprotected RP.
Incidentally, this match (as well as the subsequent 1978 WC) was covered in CL&R by Keene and Stean, and their coverage of both matches was, quite frankly, rather biased in favor of Korchnoi.
To give one example, regarding Korchnoi's blunder of 38.Qxa5 (when he clearly should have taken the KRP instead), they state that Korchnoi had probably "expected a genuine defense of the KRP" but when Spassky mistakenly played 38.g3, Korchnoi "didn't have time to make a the adjustment" in his train of thought" and thus he captured the wrong pawn.
In other words, they were just trying to nonchalantly give an excuse for Korchnoi's mistake.