Banned for MMR Boosting just now and don't know why?

It's called a "ban wave." For most of you, Riot has had your accounts flagged for a while. It's not something that they decided was happening this week. Your accounts have a history of activity on them that checks off multiple criteria that Riot looks for, and they probably decided you were on the ban list weeks or months ago. However, they don't pull the trigger on boosting bans individually, so the people who regularly boost aren't sure exactly what part of their routines flagged the accounts.

So now that Riot has this long list (or just flagged cells in a database) of accounts that are to be banned, they wait till a chosen date and ban them all. This is why there is a flood of posts on the boards complaining about being banned for boosting all at the same time. Many of the guilty parties will deny it. In some cases they may be innocent, in which case Riot can and will reverse the bans if manual review determines they were caught by false positives. In many more cases, the guilty parties incorrectly think their boosting was undetectable because they got away with it for so long, and don't realize the bans have nothing to do with recent activity. They may actually have not engaged in any boosting activity for the past month, but were caught earlier in the season. This is one of the reasons Riot doesn't ban boosters immediately and individually. They don't want the guilty parties to figure out what specific behaviors trigger the ban so that they can't find a way to reliably boost undetected.

So no, you don't have to have been boosting these past few days to have legitimately been banned for boosting.