go off at a tangent
To begin addressing or discussing a topic or topics not relevant to the main discussion. I tried to address the customer's problem, but she kept going off at a tangent and I couldn't understand what her true complaint was. In the middle of our conversation about my finances, my advisor went off at a tangent about current events.
go/fly off at a ˈtangent
(British English) (American English go off on a ˈtangent) change suddenly from talking or thinking about one thing to talking or thinking about another: One moment the professor is working hard on a problem in physics, the next he’s gone off at a tangent and he’s talking about bees.A tangent is a straight line that touches the outside of a curve but does not cross it.