Zombie threads are old discussions that someone resurrects by replying to them, after months or years of being dormant.

This thread popped to the top despite having no visible new posts since last July. Typically, what causes that is a spammer posts a comment to an old thread, causing it to pop to the top. The mods hide the spam comments, but the thread remains on top. Because there was a new reply. We ordinary users just can't see it.