What made this happen?

See attached photo - I scored on top but the loaf ignored that and burst out of the side quite violently instead.

I have a less-than-ideal baking set up - I bake two loaves at once, one gets a proper Dutch oven and usually comes out fine, the other one gets a Le Creuset Oval Doufeu. This loaf was in the Doufeu.

Because of the convex lid, the Doufeu isn't tall enough to allow the full oven spring over 30 mins of lid-on baking, so I keep the lid on for 8 mins to get the initial spring, then take the lid off and add a tray of water to the oven for steam for the remaining 20 mins. This generally works well for yeasted loaves but for this sourdough loaf it has come out looking odd. 

Does that first 8 minutes impact the scoring somehow, eg if the bread rises too quickly and hits the lid, does that make the score close up again?