Nick, like any 32 year old, knows better than to walk up to someone he's already met and ask: What’s your name? What’s your last name? Are you famous? He is not reported to have done that with strangers on the street. He was looking for a reaction, an argument. He wanted to provoke, and he provoked enough that he was asked to leave.
I think that Nick chose to be obnoxious because he was looking for conflict, and to cause upset. He may have believed that he could control emotional responses in others by trying to elicit a shock reaction ... most likely for his own amusement.
I'm betting that he was not in that mood prior to his parents inviting him to join them. They would have left him at home if he was acting that rude. He chose to be in that rude mood after he arrived at the party.
The reaction he received made him angry, and he embraced that anger after he was forced to leave the party.
"But the final social event of the filmmakers and activists’ lives was on Saturday night, when they brought their long-troubled son Nick to a party at Conan O’Brien’s house, according to a person to whom the gathering was described.
Nick, who struggled with substance abuse issues for more than half his life,
interrupted comedian Bill Hader—whom Rob had introduced Nick to earlier at the party—with a series of strange questions: What’s your name? What’s your last name? Are you famous?
Nick, 32, had been asking other partygoers the same questions, the person said—behavior that would be unusual at any party, but was particularly so at a gathering of entertainment A-listers.
He was eventually asked to leave, the person said. It was the last time Rob and Michele Reiner’s friends in Hollywood would see them alive."