Breeding/territory. Everything a cichlids does is mostly breeding driven. They claim the territory to attract mates and raise fry.
It is their instinct to do this.
Lol, I actually don't mind my cichlids digging. XD I am a scientist, so I like to observe er, 'natural' behaviours, so I just let them be - I actually like watching and seeing how exactly they dig up their pits, where and when and all that.