Are you sure that's not "the bus driver is waiting for his clock to read the scheduled departure time" or "the bus driver is waiting for people to pay their fare and board the bus" or "the bus driver turned the bus on early to warm it up"?

Every bus I've ridden in has signs plastered all over the front saying how you need to idle it when starting up / shutting down the bus to avoid destroying the innards, so I would expect there to be long horizontal lines at the start/end of runs but I wouldn't expect that to be indicative of drivers being "lazy".