If I attack spiders with multiple strategic movement left I occasionally end the battle with 0 movement. I think, but haven't confirmed yet, that it happens when you finish the tactical battle webbed.
For example I had 5 strategic movement left, entered a battle with a Ravenous Harridan and it's party. When I killed the last spider I was webbed. When I got back to the strategic map, I had 0 movement.
