Can someone provide an explanation?
Any post that does not comply with the normal posting etiquettes of this forum in particular is regarded as a troll, an advance trolling could lead to spamming, such post(s) are often deleted by the moderators and some on off topic.
You are not exactly correct, a troll tries to get people offended or bored with irrelevant diversion. If the comment is not related to the topic, it is outright spamming but for such comment to qualify as trolling, it has to upset someone. I think the right explanation have already been given by @DeathAngel, just read through his comment again for better understanding.