What is that person doing, interrupting everyone and changing the subject every minute? He is trying to commandeer, or take control, of the conversation.
You can probably tell from the look of the word that commandeer is somehow related to commander in meaning. A commander is someone who is officially in charge, giving orders and making decisions for a group such as a nation or a military. Many groups don't have a commander, but this doesn't stop some people from trying to take charge. If you commandeer something — whether it's a car or a conversation — you use force to try to take over.