rebby, I agree with Chris C., those cracks do not look like a crack from thickness, but something else. so a few questions-did you place anything inside of that bowl during the firing? Did you fire the bowl right side up, or upside down? Is your kiln shelf kiln washed? Did the bowl have anything unusual happen during its construction(warped and reshaped, dented and smoothed etc)?
As everyone here has said, the humidity which raises the atmospheric moisture takes a long time to remove in firing. I bisqued a load the other day, and actually (watersmoked) candled the kiln overnight with the lid off(bottom switch on low) putting the lid down early in the morning with bottom two switches on low-top off. I left all peep plugs out. 2 hrs later, I put the top switch on low, and then 1 hr later put the bottom on 30, middle on 20 top left at low. Slow rise til red heat in the kiln, then two bottom switches to 75 or 3/4 top to 1/2 then at red orange heat all switches to high until 06 was at 90 degree angle. Hope this will help.
I don't think anything was in the bowl.. It's hard to remember but there could have been a mug in it. It was right side up. My kiln shelf could probably use another coat of kiln wash. I am more picky about it for glaze firings. I don't recall anything strange with construction.. Honestly when I try to make larger pieces sometimes they don't work out and I trim off the top half of the piece to make a smaller one. I don't think it flopped.