I'm going to disagree with the probably were knocked up when you got them theory simply because I have a tank that was void of any plants but two that had been through a QT period and apart from the olive nerite snails that were intentionally introduced, I had no trumpet snails. From out of nowhere one day I spot a baby trumpet snail. Okay, I can live with that. Many months later and no changes to the ecosystem.. another baby snail, now I come to find five more babies a few months later. From the known one adult, I now have at least seven little trumpet snails cavorting about. PS I have MTS snails for sale. LOL
Oh, and I don't overfeed. I have a sole betta in that tank and I am fanatical about feeding him his three pellets in the morning, four in the evening and fasting him one day on the weekend. Nothing goes to waste. He is hand fed and has a voracious appetite. The nerites chomp on algae and the apple snail is content feeding on the glass as well without extra food being introduced.