Why Would I Have Maggots In My Beautiful Clean Pool? How Did They Get There? Its Never Happened Before.?

This morning i discovered hundreds of maggots in my perfectly clean pool, with no evidence of how or why. Our garbage isn’t near our pool, we are very tidy clean people & our pool yard is free of clutter or rubbish. Our pool shop guy had never heard of this before, has anyone else? We gave the pool shock treatment & removed all the maggots but we are in the dark as to where they came from. does anyone out there have any ideas or a similar story?