I'd say it's just a fluke. Arrows break. Sorry to hear about the deer though. My brother shoulder shot an antelope in Wyoming last week, didn't break through bone and it wasn't a lethal hit. We let him bed for a few hours and then went after him again. Never did get another shot off. He tore off pretty well on 3 legs, very little blood where he was bedded. It's awful feeling, but you hunt long enough, especially with the bow, you will lose an animal.
If you're not confident though, you may want to try some new arrows. Confidence in your equipment is a big thing too.