It's a bug. Adventurers that sleep on frozen oceans/lakes (not glaciers as a biome, I'm pretty sure, but actual frozen oceans or lakes. Maybe rivers with a freeze-thaw cycle too.) and under a couple other circumstances will have a "body swap" complex, where the game for whatever reason hands control over to some other creature in the area,almost always in the caverns. Once it begins, it never returns to your original body, or even to the same body twice, and every time you fast travel/sleep/wait it will occur again. Crashes are both possible and nigh-unavoidable. Rumrusher figured out how to control that, if I recall (either way it is now a default function of DFhack and entirely possible to invoke in a controlled manner), but it's still possible to cause it to happen through the bug.
I ran into this problem first while modding a race that lived in the oceans. I wanted them to live on the coasts, but telling them to spawn in ocean biomes means literally that their sites would be out in open water, and thus not exist. So, when you tried to play as them, you would spawn in the caverns under the site, as a crundle or rat or something instead of a paklara. The solution was to have them spawn in wetlands biomes and then prefer oceans/lakes over any other biome, which causes them to build along the coasts, safely and responsibly, so long as they never actually run out of shoreline to build on (then they'll "build" over open water again)