Terrier means 'digger' ! It's what terriers were bred for. I'm surprised that she hasn't shown this tendency before, but, having said that, I have a Dandie Dinmont terrier, age 2, who hasn't got round to digging much yet. Perhaps some wait until they're older. I wouldn't worry about it.My, older, poodle/spaniel crosses dig and so does the wolfhound pup; in his case only 'helping' me when I dig in fresh earth.
It remains a mystery what they are digging for, sometimes. Digging at molehills makes a certain sense; at least there is or has been an animal there; but the way some of mine dig so enthusiastically elsewhere,pausing to have a good sniff in the hole, before furiously resuming, suggests they sense something is there. Digging for earthworms seems as unlikely as it would be unrewarding.