Thanks for the tips.
I put the (full) brood box on the top for the winter with the 1/2 below. It was between these boxes I had the problem. They did not do it last year.
I have BB space, and my frames are bang on flush with the top. So I dont think incorrect bee space caused the problem for me.
I have be thinking about it, and this hive was doing it in supers towards the end of summer. Even though there was space in a super below, they still wanted to fill the gaps between frames in the supers above.
So I wonder If it is just something this hive likes to do.
Or it could be because of the shear number of bees that were in there last year. When I took the supers off, I left a part full one on the bottom, so hive was left with brood + 1/2 + super (on the bottom). And the bees could only just fit in there, every frame was packed. So I think they may have built it when I was feeding syrup in the autumn.
It was heart breaking for me (I know I am a bit of a softy with my bees). It was almost every frame, there were easily 200+ fat brood, some like big maggots, but lots had had part developed wings and legs. So some were very close to emerging. I am sad that I killed so many right at a crucial time. I expected more of a reaction from the bees, but luckily they did not seem bothered in the slightest, even when I had to scrape a load away.