Only it's not all my heroes and troops that were immobilized, but only those in one stack. The others--four heroes and their troops in two stacks--were perfectly fine, in two other stacks. Which doesn't make it sound like a match for the condition you state, but seemingly more like a bug.
Whoever is casting the Spell of Making gets immobilised. Everyone inside of the stack the caster is in can't move either (unless you remove them from the stack).
You've said, that you began casting the Spell of Making and have one immobilised stack. So, it sounds like the Spell of Making is the cause of this. Or am I missing something?