That hole has too neat an edge to be a chewing. Furthermore, within the hole you don’t see uniform flesh but what look to be gills. As with any organism, development can occasionally go awry in mushrooms and produce abnormal growth. I suspect that what you see here is a second cap growing on top of the large one, but upside down. Such an abnormality has been reported now and again in various countries with various genera. If you go to the National Library’s digitised newspapers ( http://trove.nla.gov.au/newspaper) and look up The Sydney Mail and New South Wales Advertiser, 14 June 1884, page 1124, you’ll see an illustration of an upside down cap on top of a normal cap and another rightways-up cap growing from the upside down cap.