As Alina says they're the result of current weather conditions. Digging them up won't work as they're only the fruiting bodies of a large underground network of mycelium which is really the main part of the 'plant'.
Are yours forming in rings, or are they appearing more randomly? If they're fairy rings like ours, it's helpful to spike that area of lawn regularly to stop the mycelium forming a sort of waterproof area which will result in dry patches. And continue as you are, brushing them off prior to mowing.
To be honest we enjoy them as yet another manifestation of the wonders of nature and we recapture a little of the sense of magic from our long lost youth as we watch the fairy rings appear (then once a week we brush them off and mow the lawn). 
