Dead swordtail fry,mutation or not?

Hello , I have a question. I had a pregnant swordtail ,so I put it in the breeder net the whole night.I woke up this morning to find baby swordtail fry in the breeding net( about 50) but they were dieng,some were already dead.try all had huge yolk sacs on their stomach, and some were curled up in a ball.5 hours later all of them died.what happened?