I think my adventurer got caught in the middle of a siege.
I had made an adventurer with no weapons skills, and only points in speaking, writing, and swimming (so I didn't end up drowning), just to see how long I could last as a traveling storyteller/author guy. I needed some writing supplies, and since no human towns around me had any libraries, I decided to go to a dwarven fortress to see if I could find one there.
Everything was fine for a few minutes, as I wandered around the fortress, trying to find the library, until all of a sudden my screen was filled with goblins yelling "I have defeated many fearsome opponents!". Since my adventurer had no weapon skills whatsoever, he ran down to the bottom of the fort, hoping he didn't get his leg chopped off by some goblin on the stairs.
He paced around for a few minutes, before going back up to check if the goblins were gone. I heard a goblin axeman yell "I must withdraw!", so I assumed it was safe to go back up. The stairway was splattered with blood, the walls were almost entirely covered in red, and dwarven and goblin bodies were laid all over the fort, with puddles of blood beneath them.
At this point, I decided to retire my adventurer, and check legends to see if it would give me any explanation as to why the fort was invaded. Sadly, no explanation was given, but I did get a kill count.
Can sieges actually happen to fortresses while you're in adventure mode, or was it just a coincidence that the goblins showed up when I came in? I didn't see any asterisks or anything near the fort when I was traveling.