A possibility of what happened is that Trump, through his attorney, paid off Stormy Daniels with a shady NDA. There probably wasn't actually anything eye opening here, aside from technically the adultery, which nobody actually cares about. If Trump had just left it at that, then the worst that could happen to him is that the public finds out he had an affair and that he likes to be spanked.
But Trump can't stand the possibility of the public finding that out, so he does what he always does: He lies. He says that there was no affair, and that he never paid Daniels, and that she's not allowed to say otherwise.
But if he never paid Daniels, then Cohen's payment was an illegal campaign contribution. And Trump's statement is thus strong evidence that his lawyer committed a crime. Strong enough evidence, in fact, that the FBI can get a search warrant for Cohen's home and office.
And in the course of that search, the FBI will probably find admissible evidence of all sorts of other crimes that Cohen and Trump really did commit. Evidence that would never have come to light, if it weren't for Trump's reflexive lying. And meanwhile, the public still find out that he likes being spanked, anyway.
And...to top it off...doesn't the Comey book come out next week?..