My 16 week old Ziggy does this. Yesterday, he suddenly started digging and within seconds had dug up a hazelnut. Bet the squirrels aren't pleased.
That makes me think its targeted and he's after something, rather than it just being random digging.
That said, we were on a walk where there was a children's play area with sand. We were waiting outside for the children and I thought he would dig to Australia and I'm pretty sure that's just for the fun of it.