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).



I searched for existing questions and found this:https://success.salesforce.com/answers?id=90630000000gjvjAAA 
I'm not sure if this explains my question.