Interesting question, this can be open to debate over many different things, some people debate about it being shod v unshod. Shod horses and barefoot horses are both able to land barefoot.
Most horses however, (shod and barefoot) land heel first.
But mostly it will happen if a horse doesn't have a really well developed back of the hoof, and it hits a hard spot. It shouldnt occur if the horse has a really well developed rear of the hoof. (such as good heel bulbs, good solid frog, thick digital cushion.)
I have 2 horses at the moment and one is sore in the feet and has been landing toe first, boiled it down to the fact that my new farrier had been leaving my boy in particular too long in the toe.
Found a new farrier and Im hoping that this problem will be fixed.