Biden is very insecure about his age and physical fitness, remember how he challenged trump to a push up contest, how he has to "run" out to the podium etc. he was trying to act all fit running up the stairs and tripped, several times, it was funny. in reality both of them are old people and old people are slow and fall down. hahaha the point I was making, is imagine if trump had done this? it would be all over the news 24'7 for a week.
One of the few things he has in common with Trump whose physical insecurities include his falling and graying hair, his paunch, and the size of his hands and penis.