I don't exactly remember when this started, within this past month for sure. I have my summoner spells mapped to mouse 4/5 and randomly, in a match, pressing one of them will tab me out of the game. Only ever once per match, and not every match. Mouse 6/7 don't seem to make it happen, but I don't use those as often.
The only thing I think may be the cause is have an internet browser open since the majority of the times it's happened, I had chrome open on my second monitor. Maybe it's a back/forward thing. If it is, I'll post an edit here within a day.
Edit: It's not browser based, but there is a temporary solution. Hold Ctrl, then spam your mouse buttons, the game will tab out and you won't use a summoner.