Board/LoL site always randomly logs me out. Why??

I've made a thread or two asking this already before, but have yet to receive an answer. :|

It's not too big a deal, most of the time I can just relog back in and nothing's different, but sometimes it wont let me log back in, saying my username or password is incorrect when it's not. At that point I can't do anything but wait a bit, refresh, and im magically logged back in. It can get pretty annoying.

So again, why does this happen, and is it a known issue happening to anyone else?