Why do some overweight people waddle when they walk?

Answer:

Perhaps their thighs keep rubbing together they have to waddle, it might be a natural body reaction to prevent inflammation.
Alternatively, their weight is too much for their muscles and bones and they waddle because it helps movement.

I have a friend who is overweight and is made fun of because of his waddling. When he waddles it is because of his thighs coming together and because his rear end is too big for his pants. He thinks he has his weight under control, but the waddling is still very consistent and unfortunately I have to talk to him about it because he is being laughed at.