This is a bug I encounter regulary-ish, about every 3-4 games.Last time this happened to me was earlier today. Basically, if you right click on an enemy unit this sometimes causes your units to freeze. A workaround would be an attackmove behind the unit you want to attack, but especially when attacking bases or wanting some micro in a fight this can lead to your armys quick demise, either bc. half of them is not doing anything or attacking the wrong target.
I hope these 3 clips highlight the issue: