The way image boards work is that there is a catalog. The catalog contains a limited number of threads, usually ordered by date. When a new thread is created it pushes the oldest thread in the catalog off the QResearch board.
Yeah the threads that Q drops in do fill pretty fast. Every thread locks at ~751, the baker of that thread/bread "bakes" a new thread, and everybody moves there, linked if the baker is able. Q drop is different threads, but it's not exactly random. Normally the QResearch General threads, but has posted in many other international threads.