Passengers in the 'hog get booted out and fall to the ground when a Zep loads to the next zone. Tested numerous times under different circumstances, and despite everything, my passenger would always get booted and would not travel with the zep to the next zone - in this case, from Howling Fjord to Undercity.

Could be due to the fact that the passenger is not in contact with the zep whilst being a passenger, thus not "phased in" with it or something.