LeaverBuster is either a formula or series of thresholds that trigger on a few leaves in a short span, or slightly more leaves in a longer time span. That is, 3/10 will probably trigger it, as might 4/40, or 6/100. Riot doesn't say because people will abuse it, and the stat-focused 'experts' of the game haven't explored it, because they don't particularly care.
LeaverBuster does not consider fault, blame, or excuses because they are irrelevant. It is intended to prevent your problems, legitimate or otherwise, from dragging 4-9 other people down with you. Fix your situation or find a game (probably solo -- I recommend Torchlight) where your interruptions won't hinder other people.