I don't know what it is causing your problem but I've had issues before unrelated to LoL that while uncommon, were still a bug that needed to be fixed somewhere, all kinds of bugs can exist that may even vary from hardware to hardware, including things like Motherboards. It could be something as simple as a dying stick of RAM, or instead a software issue on your end.
Could be as simple as disabling some program that's running when you want to play league, could be an issue with the OS, could be an old computer with old drivers that don't work well with some newer aspects of the league client, and some issues go forever unresolved.
It could be a lot of things basically, and I'm not sure whether it would or would not be Riot's fault.
With League I've had one issue, my computer will lose focus of the game and show something else minimizing for a split second. I don't lose control in the game but it's distracting, it only happens sometimes, usually no more than once or twice a game and often not at all.
As far as I know turning off all other programs with a window open doesn't fix it (shows desktop instead or something). I've never seen a similar issue reported by others when I've searched on Google, likely an issue I'll have to live with as long as I have this computer, but it's fairly minor and doesn't usually affect my gameplay.
That only happens for me in LoL btw, no other game has ever done that.