Dottyback Behaviour Question

I have a purple psuedochromis that is continually excavating caves under the
live rock. We have all the live rock supported by PVC so it can't come
crashing down on him. Bob (named for Bob the Builder children's books) takes
mouthfuls of sand and spits it out all over the tank. When ready to
"breakthrough" to the other side he apparently blows the sand outwards.
Anyway we see an explosion of sand and out pops Bob.

Is this a normal type of behavior for this fish? So far he has stressed 2
feather dusters by spitting sand all over them. One left his tube never to
be seen again and one dropped his feathers and is now regrowing them. The
two feather dusters Bob doesn't spit sand at are doing fine.
--
Ann R