There are situations when the "autopilot" check assumes the ice is safe, lets you walk on it without asking you... and you break in.
Indeed it is very unlikely, but in the end its all just about RNG and probabilities.
I learned that in the end of winter, when crossing open mire terrain.(zoomed in) The frozen patches are quite hard to distinct from the snow covered ground. Most of the time the character crosses them safely, the game never asks "if you are sure", but in one of 20 cases the ice breaks and you are freezing.
The ice was middle-thick(safe to cross for the "autopilot") and i had about 70 lmbs carrying with me(average middle-term journeying freight).
This occurs on threshold situations, where the ice is thick and you are heavy, or vice versa(thin ice and light character).
It wont happen when the ice is 1m thick and you crouch naked with a 40 lmb character.
What was the load you are carrying?
How long does it take to make a hole into the ice?
These are the measurements you could take to explain the case.
You said it was thin ice. Your character is not very heavy, or is he?
I assume the engine calculated a tile to be, say, 90% safe, and decided to put you there, but it was a threshold tile and you got bad luck.
However, this never happened to me when moving on zoomed out terrain. But i heard/read about that.
edit:
There is also another possible explanation: Your character was on the water tile with a boat before, zoomed out and travelled. The next time you enter the area on the overland map and you zoom in, the game remembers your last tile location and puts you there, regardless if you are on a boat or the ice is unsafe. This is a known issue for every water traveller