Your mixture is likely too dry or the chickpeas didn’t soak for long enough. If you’ve already formed the falafel, there’s probably nothing you can do this time around, but they will still be delicious even if they’ve broken apart into smaller pieces. A good way to avoid this is to make sure that the falafel mixture easily comes together in patty shapes before you cook it. If not, add in some water until the mixture easily holds together.