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.