Why do hippos yawn?
Maybe because they are sleepy?
Unlikely.
It is a display of aggression ("back off"), dominance ("my lawn"), or display ("look what a handsome fellow I am!).
On safari I learned that hippos can spread their jaws wide open, 180 degrees. That's a serious stretch! Look at those teeth!
I already knew to keep my distance. Hippos are one of the most dangerous animals on the Serengeti. Wowzer.