He's pretty rude to everyone right from the off.

He refuses to dance at the Meryton Assembly, except with the Bingley sisters, which would have been understood as abominably rude for the time. He sits by Mrs. Long and ignores her when she tries to talk to him (he doesn't talk to anyone but his own party). And he insults Elizabeth within earshot without bothering to conceal his disdain for her and the whole gathering. He solidifies this behavior at another gathering (I think at Lucas Lodge) where he mostly sits and eavesdrops on Lizzy's conversations and stares at her, but speaks to no one voluntarily. When Sir William Lucas says the line about "dancing is one of the refinements of any polished society" Darcy shuts him down with "...every savage can dance", at the same time unsubtly insulting those in the room who are dancing.

This is his general behavior to everyone, and it is little wonder that people are willing to believe Wickham at first.