cos you were AFK / leaving games previously.
When you leave intentionally (including dodging trolls), you use up the leniency built into the system for things like RL emergencies or bugsplats. It sucks when the trigger game is something outside your control, but you've also admitted to leaving at least one game by your own choice.
Dodging in Champion Select does have a similar queue wait but (as far as I know) does not count towards LeaverBuster. However, if you dodge in the last couple seconds, sometimes the signal doesn't reach Riot servers before it goes to the loading, it can look like a failure to connect, which will count towards LeaverBuster.