Sex. That's what they often do when they're very much satisfied with everything you provided them. Next thing ya know they'll lay hundreds of square shaped eggs all over the place and mostly at the base of rocks but you'll find several stuck on silicon sealant, glass, plants, etc. Not all will hatch though. Usually a small percentage will hatch and even a smaller percentage will survive to adulthood. The survival can be increased if you ensure adequate supply of pond snails for the snailettes to eat. They don't appear in the open until they are at least 1/4 inch in size.