During champion select the client forces windows focus to it 3 to 4 times, I believe this is something that should be up to the user to manage and not the scope of what a game client should be doing.
I use a number of niche programs that have issues when focus is lost during certain operations potentially resulting in unsaved files being completely lost or unrecoverable changes happening. Examples are 3DMax, Maya, ZBrush, and 3DCoat which can be unstable when tabbed away from while doing certain actions. As an example right now my 3DMax has no cursor when I interact with it because LoL claimed focus while I was in the middle of something.
I'm not saying LOL is the direct cause of the crashes but pulling focus so often automatically does increase the frequency and doesn't seem like the right choice.