Actually, this particular stumble was mild compared to some of his others - falling up the stairs to Air Force One, falling off of his bike, tripping over a sandbag at West Point, etc. His handlers know it's just a matter of time before he takes a spectacular fall that can't be explained away as "the wind" or some other lame excuse.
For those of you who are wrestling fans: I'm waiting for a "face plant" that would make Ric Flair or Greg Valentine green with envy.