That's the way the AI is set up - after a certain amount of V-hexes (random) are taken it will advance on them (at random times per formation). Since the original SSI AI code is so objective-focussed it becomes easy for the human to lead it by the nose by taking (or not) objective hexes.
Its also not very good at guarding objectives its taken, in the advance etc. so e.g. you can let it take an isolated cluster and later sneak up on it with some scout cars once the main horde has left the area.
Which is why I'm experimenting right now with getting some formations to stay behind and guard objectives rather than take part in the counter-attack, or trundle off into the distance if attacking. Guard formations will stay close to their "guarded" V-hex. Guard formations are only supposed to react to the guarded hex and possibly any fairly close to it, ignoring any objectives far away.
I have already been hit by this code - a German panzer platoon and some grunts doggedly remained in the one V-hex zone when the others all started the march to the main point of attack I had liberated. They needed dealing with individually by my subsidiary attack force.
There is some randomness in the assignment - so you cannot tell if V-hexes have guardians or not till you find some "sticky" formation(s) in the area
.
Still in early days of testing, but
if it works it should be in the next release, for the full game holders.