I'd say that's abusive. It could be worse, and we do say things out of anger sometimes. But, it was an abusive thing to say.
I would definately apologize.
Kids ask why because there brains and still growing and developing. I believe its common because that is how they figure things out when they don't have as much life experience as adults do.
They just don't understand so it's easy for us to lose patience. But, it's still wrong and you should, imo, apologize.