I guess I sort of understand at least part of this....
I've never knowingly plunked a queen excluder between brood boxes, only between brood areas and supers, so that explains why my bees have never gone into queen cell making mode above the excluder.
But isn't it frequently suggested that dividing a multiple-box brood nest with an excluder is a way to determine which box has the queen in it by seeing which has eggs after three days? (This would be in lieu of actually finding the queen.) If what you report about an excluder prompting initiation of queen cells is common, wouldn't that mean the beekeeper would also often find a whole mess of unexpected trouble in the box (es) without the queen?
And, this is a question Juhani: why would they be considered supercedure cells, rather than emergency cells? That really baffles me!
I must be unusually dense today. Think I'll go out to my bee yard and ask the bees about this.