"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 World of Warcraft!"
Person 2: "o rly?"
Person 1: "ya rly?"
Person 2: "R U TRIPPIN?"