Tangents are cool. Imagine you hold a ball and spin around and around in a circle. If you suddenly let go of the ball, it will fly off in a direction tangent to the circle at the point you let it go. And this is exactly what going off on a tangent means, both when used literally and used to describe someone who strays from the main point of a conversation

в связи с этим обалденный новый посыл: а не отправиться ли Вам по касательной)