Being an idiot.
Mike Tyson did it.  I'd like to hope Mayweather has a stop point at which he knows his interest on holdings alone can pay for all his expenses...  But when your expenses include lavish and stupid things like dropping $10 mil on the super bowl I guess you stand to fuck yourself over eventually.
But seriously daily interest on $150M is something like $45k give or take a few thou.  As long as he doesn't spend more than that, he's making a damn good yearly wage every single day just sitting around haha.