I've read of situations like this before where the ban was an accident (or not legitimate) and was overturned several days later. Make sure you write them an email to contest the ban. With the info provided, it sure doesn't look like anything is wrong. If your server has no other multiboxers, it's possible a GM didn't know what was going on, maybe he's never seen it before, and just banned you.

I've reported botters before. A GM responded with something like "I've observed so and so, and he is indeed a botter. I've turned it over to our botting/hacking team". This particular player was botting several hours after I got the GM response, so it's not a quick process, at least in this case.

My guess is that a GM flags an account for review by specialists. This is probably so that the hacking/botting team can double check, and check on the warden logs, processing running and commands being sent to the wow servers. Probably so that they can use this info to catch botters more effectively in the future. I'm sure they log the exact reason the acct. was banned, they just don't share the info.