Slitherine

My army attacked an independent region with walls, defeating the enemy but leaving some of their troops alive. My general died in this battle.

My army strength 80 sat in the province with the walled city and the enemy army of ~35. I ordered no move, with the intention of seiging.

Next turn my army returned to my home territory on its own.

I cannot imagine what led to this. Anyone offer insights?

There is probably an explanation.

If there isn't already an icon in one of the regions involved with a message in the tooltip explaining what happened, you can dig into the events of a turn further by opening the message panel from the lower left of the screen (when the other panels are closed). Try changing the Message Importance filter along the top of the panel to Average or Low and see if a helpful new message icon appears. You might also need to change the Diffusion filter to Not Involved to see the events involving your neighbour, although you will likely want to switch that back before you continue with your game so you get your own news.