[okareaman]

There is a word for that: threadjacking

[Dylan16807]

When it's a top level comment, I would not use the word threadjacking, and I would not say it's very problematic.

Replies need to connect to the comment they reply to. That's not the issue here.

[TeMPOraL]

Direct replies. One of the best features of tree-based discussion is that "thread-jacking" not only isn't a problem, it's beneficial. A discussion tree can handle subthreads and tangents just fine.

[coldtea]

There's also another term for it: casual conversation.