I have also noticed at gamestart or after restart when pausing the game, that two vehicles are dispatched within short time. This is normally related to the fact, that the sceduled time was around the time the game was started or restarted. Then the game tries to bring the vehicle, that just should have been send out and directly after that the next vehicle.
But this should not appear randomly during a running game.
Hmmn, for me it seems to happen throughout the whole game; any depot of any transportation type whether underground or above ground would once in a while dispatch two vehicles at the same time. And I honestly have never seen this before this patch otherwise I would've brought it up. Only after this patch, I have noticed this.