Well, the official explanation came from SRC Terendil;

No SRC was involved other than to stop the rogue counselor. The Counselor in question has had his account permanently banned for this. It does not matter if he was hacked or not as he is ultimately responsible for his account.

Catskills Shard extends its apologies to any affected player.

And here’s what some of you came up with. By far the most popular explanation was variations on this:

They figured that everyone macroed at one point or another so they just decided to punish everyone for back crimes.

Other theories include:

Population control has been introduced into UO. Maybe the GM showed up to kill them and remove their chars from the server, thus creating a less lag free experience for everyone as these people would probably quit.

These people all willed themselves into jail. A few of them were thinking “Im bored” and then in a mind over matter incident, the server transported everyone on screen to a more interesting spot. One with walls on all sides.

It was a cruel experiment to see how players act in twisted conditions. Stuck in a jail room with 40 other people all shouting and screaming? In RL, that would be torture.

I think they just wanted to see how many corpses could decay in a single room.

It was some kind of exploit… if you piled 12 mushrooms in each corner of a jail cell, a random castle would be yours!

And my favorite explanation:

The smurfs were jealous of all the attention you gave SRCs last week…