If it's not blocked veggies or Planet Eaters I have no idea. THere is a natural repulsion to all bots, so the only way they can overlap like that is extreme forces. The only forces I can think of that would do that are planet eaters and blocked veggies.
Open the console for one of the bots and type ? .fixed
If it returns 1, then the veggies are fixed and hence blocked. It's possible for veggies to mutate this.
If it returns 0, then I have no idea.