The AI rally has been pulling money in one direction for months. Funds are chasing it. Retail accounts are piling in. The question most investors are asking is how much exposure they have. Few are asking what happens when it turns.

Mark Cuban has seen a tech mania up close before, and he thinks he is watching another one. The billionaire investor now says the artificial intelligence boom has the makings of a bubble, and he wants investors to keep some powder dry.