Is there some reason quarries set off their charges at seven a.m.?

Like most factories (and that's pretty much what they are), they have a schedule, and stick to it. They are probably set up to know how much stone they can move out of the mine in a day, and that's how much they blast away. They probably drill the holes ahead of time. Then they start work at 6, and lay the charges first thing, then blast at 7. It would be really dumb and unsafe to leave charges out over night. Anyone breaking in could set them off.