Thread derailing occurs when the main topic of a thread is lost due to posts which change the subject and focus of the thread.
Just about everyone who has ever posted in a forum has been guilty of derailing a thread, but it is tolerated to a degree by the general population as long as it wasn't intentional or if it is short lived.
Forum trolls thrive on derailing threads and are a pain in the ass because they break topic all the time with no purpose other than to break the topic.
Aika is known to derail threads simply by posting in them, which causes forumers to post sexual comments and more or less totally hijack the thread. She doesn't mean for it to happen, though.