Grouping a result of collective movement?

I noticed over the weekend that I was ending up with unexpected grouping configurations.

I would box some units, or individually select a few, and then would send them somewhere.

And all of the sudden, they became a group.

In some cases, would happen with engineers.

In some cases, would happen with military units.

Seems strange.

And it wasn't consistent either.

Shouldn't I be able to "cluster move" without creating a group? 

Anyone have thoughts on why this might be happening?