Blow Outs in the slashes

As I am new here I posted my first two loaves of sourdough under the pictures forum the other day. I am not sure where to post this question.

My loaf tonight was a boule, my first. I made three pretty slashes, (at the time) and slid it off onto my baking stone. I had a huge blow-out in one of my slashes. Did I not let it rise enough on the final rise or did I slash too deep? Any thoughts, advice?

Thanks everyone, you are all great and I am learning so much!

I cannot get the pic to load but here is the link!