In my opinion, if you and someone other than the OP are going back and forth in a discussion that adds nothing to the OP, you are dominating the thread and derailing it. To avoid this, pay attention to the conversations you are having and ask yourself if you are still adding to the original intended discussion. If the OP were to return right then, would they be able to jump in as well or is it now really just a conversation between you and the other person? Also, ask yourself if maybe you should take it to PM rather than leaving it on the question.
If you are going back and forth with the OP and it is adding to the OP, I don’t consider that dominating in a bad way since you are helping the OP. Some people may still see this as a bad thing, but I think it’s ok when being done to help answer the question. Sometimes such a thing is necessary, such as working out a chemistry or math problem or obtaining more information to give a better answer.
To me, dominating alone is not necessarily a bad thing. Its more about the intent and effect on the intended discussion.