Was it just the warning pop-up (where you have to type 'I agree'), or is it actually a leaverbuster-issued penalty?
If it was just the warning, that will pop up for anybody the first time they leave a game, and functions as a warning that further penalties may occur if you continue to leave games frequently.
If it was an actual penalty, that means you've left games frequently enough in the recent past to get a penalty. You don't receive a penalty from one leave. If this is the case, then the punishment is a cumulative result of all the games you've left/afk'd in, and it just happens to be that the one instance where it was out of your control was the last straw before the penalty. If you didn't have a history of leaving or going afk, you wouldn't have received any penalty at all. The system is built with a little forgiveness specifically for cases where a player has occasional connection issues.
Note: The game has no idea what caused the leave/afk.