It’s the bees.
Squash is a member of the Cucurbitaceae family, one of the most difficult vegetables to save seeds from. Cucurbitaceae have both male and female flowers and lots of wild, inedible relatives. Cross pollination is what Cucurbitaceae want to do. If you want to save seed and you take the precaution of taping up the flowers, bumblebees and solitary bees can chew their way through the tape to get at the pollen. In short it’s really easy to breed a freak Frankensquash or Frankencucumber, which can actually be toxic.