I think they grow outwards in a cirlcle as it the underground mycelium spreads out to find food. Subsequent generations of fungi grow only outwards, because the parent generations have depleted their local nitrogen levels. Why the appear in grass, im not sure

thanks nickyt08
not far away from me is a 1/2 circle the rest i presume under the tarmac pavement

Yes, Nicky08, that's the way I understand it.