That's always happened to me on this site. If it is a thread I haven't clicked on in a while, it takes me to the start of the thread. Only this site have I seen that happen. Like maybe after a while the cookie (or whatever tracker keeps track of the last post in a thread you got to) falls off or is gone, so wherever I was in a thread, after a while that info isn't there so it takes me to the start of the thread. But it has to be a thread I haven't been to in a while (like an older thread that got bumped or something).