You don't know what preempted this situation. It is quite possible that the gentleman's driving was bad enough that it needed to be addressed. Perhaps it was dangerous. Perhaps the cashier had witnessed a near miss. Perhaps the cashier had seen someone come close to getting hurt. Customer or no customer I would have spoken up if calling attention to the situation was due. As for the following customers being treated rather bruskly, it could be possible that the cashier was riled up from the exchange and hadn't had opportunity to calm down. Cashiers and other people employed in the service industry are people too. The customer isn't always right. Sometimes situations come up that can't or shouldn't be ignored. If this man's driving was so bad I am glad that someone spoke up about it instead of ignoring it and keeping mum. The other explanation could be that the cashier was having a really bad day with a really difficult personal situation bad enough it was affecting them. Yes, the behaviour may have seemed bad on the surface but we don't necessarily know the triggering factor to it. If it were me, I would have asked the cashier if they were okay and encouraged them to take a moment to breathe.