It is a parental/spawning instinct and you may or may not even have a male and female pair.Some cichlids,like my red devils do it without the presence of a mate in the tank.
Yup, totally normal. My pikes have cleaned all the sand out from under their log, now it is barebottom under there and there are piles of sand around it. Most cichlids I have kept, have dug holes to some degree. Some get more of a kick out of it than others.