The tail going into ground effect is the reason given by our training department on the 737 which has always made sense to me as the tail is getting close to the ground (especially on the 800/900) and we need to keep the tail going 'down' whereas the result of ground effect is to stop that happening.
I don't understand why 'hydraulic controls' discredit this theory as all that needs to be done as one approaches the dead band is increase the elevator deflection.