D.B Cooper incident - What caused the tail to move after a jump?
The "sudden upward tail movement" requires nothing more than a sudden unloading of the tail by removing that 90+ kg of man, parachute, and money.

Even with an aircraft as large as a 727, suddenly removing 90+ kg from the tailcone airstair would result in a pretty noticeable shift in trim requirement, with additional tail downforce required to restore level flight.  This is especially true when flying at the low speed Cooper had demanded (to allow a more-or-less safe jump).