I can venture a guess, with the caveat that this is unsubstantiated.
The games runs on a heartbeat, meaning that as you are moving and not paused, there is a great deal happening behind the scenes. Lots of routines being checked, battles being fought and resolved, AI spotting other units, decision trees being referenced, and everything that can be checked is being referenced to determine if it the interval trigger is tripped. If you happen to "Save" when a process behind the scenes is partially completed, this may cause a corruption.
Now, should the processes be resolved before the save is initiated? Yes. I think generally they do. However, with larger more involved games (late game play), this has a possibility in Warband not to always work as desired.
Generally it is advised to save the game when you are in an Inn, as all background processes have stopped at that point.
While I am guessing about the processes, and I am sure a coder who is much more knowledgeable about such things will jump in to correct me, I am very confident (based upon community feedback) that saving while in an Inn is a great way to avoid corrupted saves.