Why did this sourdough have such a huge amount of oven spring?

https://imgur.com/a/RWUGIRv

As you can see it kind of opened up a lot more than I'd imagine. I did a cross on top with the usual depth of scoring that gives me good results (about .25", a little more). I have underfermented and underproofed and usually those just dont rise adequately, but this exploded, so I think its something else. I didnt see the crumb or eat it because I gave it away. Thanks!