You know, I think it's kind of weird that all the daycare children are so close and spend years together but their parents never become friends or keep in touch. Sometimes a popular child leaves and the others miss them so much for months but sometimes a child leaves and the others never mention them again. All personalities are different in children and adults. You obviously have very social people at your daycare.
The first thing that crossed my mind was that I hope you have enforced all your rules consistently and haven't let anybody have a special favour. If these parents are all talking amongst themselves about your daycare maybe it would be best for you to be there so they don't start comparing notes!