It sounds like the soil around your Brussels sprouts wasn't firm enough or they didn’t have enough support. When the buds mature in an open manner instead of nice and tight it is called ‘blowing out’. Brussels sprouts like the soil to be nice and snug so when you plant them push all around your seedlings with your foot to make the soil firm. Ideally plant them in a sheltered spot so the wind won’t snap the roots, or make sure they have some support to stay firm in the soil. This should ensure you have nice, tight balls rather than loose, open ‘cabbages’.