Basically this happens with soviets because the AI gets "Stuck"
I thought they addressed this problem but to no avail.
The reason it happens is that if you make an upgrade in the armory while you have a unit that you are upgrading in production and the armory finishes first, the unit won't produce but it will be stuck on the que. The AI is not smart enough to cancel it, therefore blocking production in their buildings. Which means they have to basically call in units to fight as a last ditch effort.
Example:
You are making a Strelky, but your Red banner upgrade finishes about 1/2 a minute before your Strelky hits the field. The Strelky won't be a Normal OR Red banner, instead it will sit on the que until you cancel the order.
The AI can't cancel the order. It sits in limbo.