I observe the same thing, but I interpret it differently. This is because this happens to me even more in conversations of the more... acoustic variety. I interpret it as more of a general resistance to spin-off discussions. In an in-person conversation, if you have a spin-off discussion, you need to stop the existing discussion. This is often permanent. For this reason, if someone wants to talk about a particular thing, they will need to stop spin-off discussions.

On an internet forum such as this, that's unnecessary, but it's a habit that a lot of people have.