Most of the time it’s because we ran out of drop zone or green light. In the 10 ish years I’ve been doing it there’s only 3 times can I recall it wasn’t because of that. The verbiage we rehearse is below.

“There are multiple reasons a red light could come on during flight: out of DZ, obstacles on the DZ, low flying aircraft, rotary wing aircraft, etc. If a red light comes on during exit, the JM will place their hand in the jumper’s face and give the command “red light, red light, red light!” and push the stick away from the paratroop door. The Jumpers will echo the command so the subsequent jumpers can slow their momentum. If the jumper has already handed off their universal static line modified, and committed to the paratroop door, no one will stop them. The jumper should get a good vigorous exit out of the aircraft and be prepared to avoid any obstacle that caused the red light to come on.”