Honestly, you COULD say they acted inappropriate and that would be kind of true but I didn't see it that way.
Buffy left town without saying a darn thing. Not 'see ya later', not 'I'll be back', not even 'goodbye'. She just left.
And they were going to have a nice, small get together actually. But, and this is my opinion, I feel that it grew to such extremes because the Scoobies [and Giles, maybe] felt that that would be better than having awkward conversation at the dinner table [not to mention having the elephant in the room: Buffy just upping and left].
That's how I seen it as, anyway. That way, they didn't have to pretend that everything was fine and dandy [even though Buffy and Willow and everybody else DID have a big blowout in the end].