why does Brutus treat his wife so eccenticly in the second act?

Eccentricly? If that's what you mean I don't see it as eccentric. Brutus has been keeping secrets from his wife; he doesn't do that. They love each other and are partners; he shares his feelings with her, but suddenly that isn't happening and she knows something is wrong. This is definitely a case where Brutus is acting out of character, but I don't see it as eccentric. Brutus tells Portia that he wishes he was worthy of her............. that might lead us to believe that his neglect in talking to her comes more from shame than anything else.