The passenger who was short on his fare knew that before he boarded. He should have informed the driver before putting any money in the fare box or tapping his Ventra Card. If the driver kicks him off, just gets off with no problem. It's the passenger 's responsibility to have the proper fare.
That said, I think the driver possibly overreacted. I say possibly because I don't know if this driver had a previous encounter with this same person who may have been trying to get over. This is the only reason I can see the driver kicking him off the bus and not letting others pay