Cendre, I've experienced this too - but only when I was beseiged and I sallied from the town. I would start out in the standard position (me just inside the walls, the army in a line a fair distance from the walls). From the moment the battle started, the entire enemy army would move right (from my perspective), across most of the map, and settle in the northeast corner, waiting for me to come and get them.

Which I then gleefully did. Romans rule on the open field, rarely would more than ten or twenty enemies survive (okay, this is on M/M, but I'm a beginner, so...).

The one time when I was "properly" beseiged (in 1.2) was when the Spanish attacked one my settlements (which was defended by 2 Hastati and 2 Town Guards) with a large stack. They built rams, tore down the Palisade (no better defences yet, at that point), made a beeline for the central square, and slaughtered my defenders. Okay, I took quite a few of them with me, but Hastati and Town Guards won't stand up to a huge amount of pain before they collapse... so the AI can seige properly, but perhaps it only does it against low-level walls and/or small garrisons...