Thats because you only have one big piece of live rock... and too much sand

I've always done a little sand, then all my rock, then more sand. I've always run a DSB, as in 4-6" worth with no problems. I've never had issues with sand creatures digging down all the way to the bottom, or even a little ways and causing the rocks to shift. The first layer of sand was to prevent the rocks in some ways from cracking the bottom pane of glass, and then the second and final layer just filled everything up and looks more natural to me.

Good luck with whatever you decide!

If you're going to use sand, put it in first. During the cycling of the rock, lots of the critters will leave the rock and move into the sand. Stability isn't an issue, just seat the rock well on the sand and it will be fine. Any detritus that does fall on the sand will disappear soon enough, and will help establish the sand bed.

I have always went sand then rock, but I make sure that the base structure of the reef is pushed well into the sand so it is touching the glass.

Personally I love sand beds, to me BB makes a tank look too much like a fish store sales tank. In the end to each their own though eh? :)