Hi, I have 4 yellow tails that have all decided to do the same thing... it started as one and I posted the same question because I thought she was going to lay eggs but.. the rest of them started dragging rocks around and fanning the rocks away and stuff too, digging down to the glass. (very fun to watch)
So I am thinking they're just digging thier homes, an AA member told me this as well. They defend their "homes" pretty aggressively too.
BUT.. I do not know about the mating habits of these fish.. so wouldn't it be cool if they were laying eggs!