Dogs dig for a variety of reasons.
It's fun.
They are bored.
It's hot, and they uncover cool earth.
To den.
Bitches dig to make a place to whelp sometimes as well.
You have already gotten a lot of good advice....
Keep them busy.
Filling the holes with feces DOES WORK, but many times they will just dig elsewhere.
I learned a long time ago to fence where I don't mind dogs digging, and not to worry if/when they dig in their own areas.