Why is my bristlenose pleco making a hill of sand?

[charlie1]

I have two BN plecos (one male and one female) in a 20 gallon aquarium. My male is about 2 and a half inches in length and he is in the cave digging in the sand. He then pushes the sand out of the cave and puts it in a hill. I am very confused. I have owned him for about 4 to 5 months and have never seen this before, I know plecos like to dig, but the whole hill thing is weird.
Water perameters are normal
pH= 7.2 ammonia= 0 nitrites= 0 nitrates= 10 temp=79 degrees
I also have driftwood for my BN plecos. So can someone tell me what is going on with the male?

[Red]

Its just normal behavior, they like to go where they like to go. Nothing wrong with that, might even get some eggs from the pair.

[Lady Hobbs]

Mine did that too. They like to dig holes and lay in them.