A troll needs attention to his trolling, so whenever he is in a lengthy thread arguing only with one or a few posters, the troll is likely to break out of that thread and start another thread that will be one way to evade the first thread and also draw more attention from more people.
The new thread usually contains nothing but antagonizing material regarding the poster or posters the troll has been in contact with in the other, ongoing, thread. It is usually filled with personal attacks, ad hominems and usually in "bullet form" where the troll will list items about the other posters in a summary form and post it in a new thread.
This is a sure sign of defeat for the troll, where it is obvious that he can no longer deal with the reasoning and logic of the ongoing thread and the only way to "get back" to his "opponents" is to start a new thread that contains nothing of the topic and content of the old thread and obviously is also off-topic for the group.
Antagonizing threads can also be unrelated to an ongoing thread and just be the trolls method of getting attention if he hasn't received much lately.