At a guess they are on a bit of a flow and are uncapping in order to extend the comb out further. It's good if they uncap to do this - I've had some frames with 2 layers of cappings.
Or perhaps they are using the ripe honey for feeding purposes.
Whatever is happening, it is best to have 80-100 % capped for extraction. They will cap when they are ready and the way your hives are filling up, it sounds like you might have to dust off some honey jars.
I have also noticed in a big flow that they capping won't be done till the flow starts to ease up a bit. In this case it is handy to add supers as you have done.