D.B Cooper incident - What caused the tail to move after a jump? In the Wiki page on "D.B Cooper", the presumed moment he jumped out of the aircraft is described:


  At approximately 8:13 p.m., the aircraft's tail section sustained a sudden upward movement, significant enough to require trimming to bring the plane back to level flight.


This was replicated in a test:


  In an experimental re-creation, Scott piloted the aircraft used in the hijacking in the same flight configuration. FBI agents, pushing a 200-pound (91 kg) sled out of the open airstair, were able to reproduce the upward motion of the tail section described by the flight crew at 8:13 p.m


What was the likely cause of this? My initial thought was it was his body striking the tail, but that isn't mentioned and doesn't seem to be what happened in the test.
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).