"R U TRIPPIN'?" normally accompanies O RLY
? and YA RLY
! as an inquiry of someone's mental state. It is most commonly used for additional clarification, but could also be used after one participant in a conversation says something of questionable relevance or of a strikingly unintelligent nature.
Person 1: "Hey, guys, let's play DOTA!"
Person 2: "O RLY?"
Person 1: "YA RLY?"
Person 2: "R U TRIPPIN?"
Person 1: "NO WAI!"