Only Mayweather and USADA can answer that. I know that random testing is done in random locations like home, the gym, while on tour maybe... But testing is always done immediately after the fight, likely in the presence of in this case a Nevada State Athletic Commissioner... My best guess is he was "being safe rather than sorry". That was the last opportunity he had for the IV before the post fight test.
Didn't Floyd weigh in at 146 for the Pacquaio fight? You would think if he was having to dehydrate himself that much to make weight, he would have just stopped at 147.