"Bundle" came from a page and picture in the classic children's book, "Danny and the Dinosaur." There is a page where the dinosaur is carrying an older lady - who has her packages from shopping - across the street. His hands are cupped and she sits in them like one would a chair.
So, she is holding her bundles and the dinosaur is holding the lady's bundles. The dinosaur had asked if he could help her with her bundles. So, the double entendre is that her bundles are her backside, which for many reads caused roars of laughter with a toddler, who then inherited the name, "bundles" and also "bundlecakes."
Bundles is slang for bum, and cakes has double meaning for what a diaper is used to contain - but also for "cake," which a child sometimes is affectionately called: cake.