what causes "foaming at the mouth" when a person dies of a opiate
overdose?

I have not done any research on this subject but can take a guess since
I have had to deal with several people that died while I was on duty.
When people die, all of their normal body processes (including muscles)
stop working. All of the urine that is in the bladder comes out.
There is a a muscle that keeps the fluid (including stomach acid) in the
stomach when a person is still alive. However, when people die, that
muscle stops working and the stomach acid comes out of the mouth (foaming
at the mouth).
If a dead person has recently eaten before they died--vomit (instead of
foam) would come out of the mouth.