old people

They've accumulated so much experience and wisdom in their bodies that if they don't let some of it escape, they'll explode, much like a pressure cooker. The mouth falling open is a safety valve to protect all the younger folks around them.

What Butrflynet said.

But sometimes their mouths fall open in utter astonishment at the stupid questions that very young people ask.

Or to re-sit the dentures after the glue's worn out.