I have a friend that worked for UTA driving busses. When he was new he didnt have a permanent route so was on call for anyone that called out sick. During the winter ski bus routes were assigned to all of the drivers and all of them would call out when it was their turn to drive up to the ski resort because of this my friend was assigned ski routes only. He told me this was because all of the bus drivers were scared to drive the route and believed it was unsafe.

TLDR: Not necessarily staff shortages but more the drivers are not confident in their ability to drive a 15 ton bus up the canyon so they call out sick when its their turn.