The outfit is the result of a mostly friendly game played by during practice by the Cardinals. They try to lob balls into a garbage can and the winners get to choose a game day outfit for the loser. That's how cornerback Patrick Peterson wound up dressed as Buddy the Elf last season.
It's probably not a great sign if your starting quarterback loses that contest, but it worked out ok this week. Palmer had a massive game, throwing for over 300 yards and three touchdowns, leading the Cardinals to a win.
Maybe he should dress as the Stay Puft Marshmallow Man every game. Though, that swishing noise when he walks would drive George Costanza into madness.