Q1: When I change the owner of an Account record to UserA, the associated Opportunity record's owner is set to UserA automatically.

Is it a native behavior? (I checked all the workflows and account trigger code but I don't see any changes to opportunity.)

Q2: Follow the scenario of Q1, even though the opportunity owner is changed, the opportunity trigger is not fired. How is it possible? (I have debug log in my opportunity trigger I don't see the log printed).