I made the bundt cake as cupcakes and they tasted delicious - we're perfectly cooked after 20 minutes. But they sunk in the middle. Any ideas why?...

Thanks!

These previous threads on the topic have a lot of suggestions for future reference: https://food52.com/hotline/6380-what-causes-a-cake-to-sink-in-the-middle-for-the-last-6-months-any-loaf-type-cake-has-been-sinking and https://food52.com/hotline/9287-what-is-your-best-trick-to-keep-a-cake-from-falling