If a vehicle moves off the table edge even partially then any models onboard may count as casualty with the vehicle depending on the scenario.
Models may disembark at any point in a vehicles move during their turn. Place the model where you would like to disembark then continue with the vehicles move.
If the vehicle thrusts at any point in its move then models disembarking that turn count as jumping from a thrusting vehicle even if they disembark during gas moves.
Heres the gorkamorka rule books