Likely a post was sent through the anti spam mechanism, I.e. Pasting in a reply instead of typing it, tripping our “boy you sure do type awful fast for a new human user” protection and rejected by a mod or admin. This would block the user indefinitely.
In other words rejecting that post by a new user will auto block the user even though it does not explicitly say that. But possibly should. It is unclear what to do with a new user who trips this detector and has their first post rejected.