The separation of the veil from the cap to expose the gills of a mushroom cup is a major component of quality. This quality parameter is judged at harvest and during packing operations and should determine the market use that the common Agaricus mushroom will be directed towards.  Maturity primarily, but temperature and humidity during production all play a role in whether the cup is closed or more open as expansion results in veil separation.

If closed at harvest, a sensible possibility is that elevated temperature, lower humidity (causing water loss) and/or age would cause veil separation and increase in cup openness. There is not enough information to be definitive; however, the degree of browning and deformity of the caps evident in the images is suggestive of advanced maturity of the mushrooms during distribution which is, of course, linked to temperature and time.
                                                                                                --Trevor Suslow