There's no way for the client to determine whether you were AFK kicked, or you got disconnected, so both resort to giving a leavebuster penalty.
As harsh as it may sound, if the problem occurs frequently, I suggest getting better internet or a better computer. A fresh install of LoL will help too.
Low priority happens when you leave champion select or games many times, after you will play some games you will get the normal priority again don't worry :D
As harsh as it may sound, if the problem occurs frequently, I suggest getting better internet or a better computer. A fresh install of LoL will help too.