Here is a note from a Riot employee:
"Hello again Ultimate,
I've looked through your penalty history and through your 2-week suspension back in December.
Unfortunately, your account did escalate properly due to the negativity that has been shown in your matches with a 10-game chat restriction > 25-game chat restriction > 2-week suspension.
Since your 2-week suspension was applied your account did not deescalate to a 10-game chat restriction properly, though our IFS (Instant Feedback System) believe it did. I cannot go into detail of what qualifies for a deescalation.
We did notice that - that 10-game was incorrect and we had to make a corrective penalty of a permanent suspension.
I know it's really difficult to swallow this entire ordeal, but please know that we've made this decision based on the history of your account and this is truly the end of the road for this account.
ᕕ( ᐛ )ᕗ ʕ•ᴥ•ʔ
Good Looking Detective"
So there you have it. You can be arbitrarily banned at any time if you receive a chat ban. They can just say the system fucked up. Cool story bro. I guess he can't "go into details".