As Loren mentioned there are engagement ranges, so perhaps that's why they held back. That's also why the Overmind pulled back, the AI can pull off these little tricks where we as players cannot
No--I don't think the other dreads were holding back. Rather, I think they were failing to close. Dread #1 is in range, it sits there and fires. Dreads #2..#4 try to head towards the Overmind but the direct path takes them through #1, they can't go there. They have to move off to the side to go around.
Meanwhile, the Overmind moves back. Dread #1 responds by moving forward. Now #2..#4 see a space to move into, they head back towards it. Dread #1 is in range, it stops. Now the path is blocked, the others must turn aside again.
The result is the other dreads are tracing a zig-zag path which slows them down a lot--they are never able to close to engagement range.
I have yet to see a game of this sort where you don't get severe traffic jam problems in some cases. The size of dreads makes this problem much worse. The problem here is that figuring out how to get all the units through a space is a very complex programming challenge, yet it must run quickly--prohibiting solutions which attempt to solve it globally. Furthermore, the nature of the engagement means that the solution keeps changing.
While it would look ugly I think the game would behave better if the footprint of a dread was reduced. Other than that the only thing I can think of is to give units a considerable preference for not being in the shadow of anything with a weapons range equal or greater than theirs. If the terrain permits spreading out, do so. This generally happens with lesser units in armies but doesn't seem to work for dreads.