With many years flying Boeing's finest, it is a strong effect that is very noticeable. You pull a rotation, and it stops at about the 10 degree mark. You quickly learn to give it another tweak approaching 10 degrees and leave it at that. That will keep the nose smoothly rotating up through 10 degrees.
I had assumed that it was the tailplane approaching the ground and running into compressibility effects of the air between the tailplane and ground, which at about 10 degrees are quite close. It may well be interaction off the flaps- I don't know.