It may be a response to specific circumstances like carrying blockade gear, announcements made by the people in question or past crimes: https://www.gesetze-bayern.de/Content/Document/BayPAG-17

I want to believe that there was a "specific action". An explicit announcement to block a road / airport / whatever.

Unfortunately, it is cheaper for the gonvernment to detain people than to send police to prevent it shortly before it is going to happen.

Also, the ruling party in Bavaria is mainly controlled by the opinions people exchange in pubs over a couple of beers (one beer being a quarter gallon), and there common sense isn't considered that much.