Tabbed out of the game

Lately (in the last 2 weeks or so), I've been regularly tabbed out of the game randomly during a match.

First I thought "Maybe I'm doing ALT+TAB by mistake since I use ALT+Q/W/E/R to level up abilities". But yesterday, as Garen was shoving his lane (i said lane) into my turret (lol), I got kicked out again but this time I was even touching my keyboard!!!!!! Result> he started attacking a bot (me) for a few seconds for free and 10 seconds later I died to his ult (my bad, I thought I had enough health to survive).

I lost that game coz I suck but it's not the point.

Point is, unless I press ALT+TAB, this kind of thing should NEVER happen.

Thanks