Banned for Hacking

So I haven't played league in 6 months to a year, roughly. Last night my friend convinced me to get on and 1v1 him, so I decided why not, I'll do it. I tried logging into my account, and it said my account was permanently suspended. Confused, I checked my email and it said I had been banned for use of third party programs that interacted with league. There was only one other League of Legends related email in my inbox, one that said there was "suspicious activity" on my account, and that a password reset had been issued to keep my account safe. Sure, no problem, thanks Riot for looking out for me. Now, the confusing part about all of this is that while I hadn't played in a looong time, somebody had been playing on my account, judging from my match history. This shouldn't have been possible, since a password reset was issued, yet they were playing on my account. I did allow my friend to play on my account over a year ago, so I thought maybe he had remembered my account information and hacked, but after questioning him, I now know that he hasn't. I did use MK LOL, the skin-changing thing, but I used it back when it was still allowed/not bannable, Iirc. This is all the relevant information that comes to mind at the moment, but I'd like someone to please shed some light on my situation, if possible. I'm not an idiot, I know that permanent bans for third party programs are extraordinarily unlikely to ever be lifted, by any game developer, for good reason. I'm not even really a league player, but it was certainly alarming seeing that I had been banned. So again, if anyone could help me figure out where I went wrong security-wise, that would be great. Thank you!