There's the risk of grounding from the vehicle to earth by a disbarking passenger, as well as of ground currents within 100m or so of a high-voltage downed line.
Mitigating those conditions requires specific actions and measures. Ensuring compliance by all passengers aboard the bus is a challenge. The lesser risk might be having them remain aboard. Even the option of carrying or assisting passengers from the bus could risk grounding. Consider that passengers might be old, young, handicapped, non-English speakers, or otherwise have difficulty understanding or complying with instructions.
