Ahhhh digger. Yes. Digging is a daily occurance in our house.. I have a few theories. But these are only based on Leo's habits..
A) cold after a walk- this is particularly managed by us as Leo still struggles with a hip injury after falling down a dyke. If he is cold he cannot get comfy and appears to dig when he's trying to find a comfy spot. Leo has a drying coat and this dramatically reduces his chances of getting worked up after a walk.
zoomies. Leo still has daily zoomies... These always contain a certain amount of digging in the carpet.. we've managed to stop him digging in the sofa by placing a loose stair gate over the spot he is digging whenever he does it.. if I was that concerned about him digging the carpet I would use the Stargate to deter him from that too but he's never caused damaged to the carpet so I tend to just ignore it.
C) tiredness!!! Still to this day our leo suffers with tiredness. Before he settled to snooze msot evenings I can guarantee he will atleast have a little scratch in the carpet.
I read somewhere once that dogs dig to find a comfy spot such as wolves would have done in soil many moons ago so part of me believes its an inherited instinct. However leo definatley has a habit of it now too. Like I said, best thing we've ever done is give no attention to it, but move something in Leo's way of his chosen digging area... So may be not much help but I feel your pain