Pure Speculation
It's been a long time since I flew the 737, and then it was only the classic model.
I don't remember any pitch problems when pulling the power off at landing. Other times it was noticeable and trim was immediately necessary.
I have heard that WN flies their approaches at less than full flaps (40) to save fuel and block time.
As I recall, if the flaps move from 25 or 30 to the 40 position, there was quite a "balloon" effect, and you had to push the nose over to counteract that.
Maybe the captain called for the extra flaps and pushed over at the same time.
Possibly some current 737 folks can comment on this wild speculation??
Regards.