Some birds are just predisposed to becoming egg bound. This is a problem that creeps up more often in hatchery breeds. I've only had one bird egg bound and that was because she always laid in the exact place every day and help her egg cause she was kept away from it. But I've treated many birds that have been egg bound for other chicken owners.
One common cause of being egg bound is from not having the proper nutrients. You must have a level of calcium and vitamin D and protein to make the parts work right. I don't recommend coral calcium that is often sold in farm stores. It only works when taken into their gizzard and worn down slowly. Calcium carbonate or limestone is the most easily digested form of calcium for all birds. You can buy a 50 pound bag of Calcium carbonate to be mixed in feed for under $4.00 a bag. It is tiny little granules, but not quite powder.
But you also need to be sure that they have the other minerals and vitamins to balance that out. Free ranging is the best way to achieve this.
But as I said sometimes you just can't prevent it in some birds.
It's a very sad thing.