1. I/we are either bored of the thread, or couldn't help being dragged up in another sub-topic that came up during the discussion, and got carried away.
2. Had somethings on my/their mind to speak, but ended up creating another topic, because of the lack of relevance to the current one.
3. May have originally wanted to think of something relevant to say, but it came out wrong/implies other things, and a derailment starts
These may not always be the case, but these are definitely mine.