I hadn't considered that sort of reply, the mouthy make it personal one. My guess is the personality type you're dealing with, the mood they're in that day, and the anonymity the internet provides. The same conversation in person would flow differently, or they'd simply keep their mouths shut. Either way it's OT and or a bit rude, in some forums it may even draw a moderator response. Am not referring to those who word it constructively.
These are folks we all have to work around. Sometimes you even have to pretend that they never replied which helps diminish and obscure their comments. Or the short curt response.