Cracked Wheat Loaf

Toast
Yesterday I made Rose Beranbaum's Cracked Wheat Bread, everything went well until I slashed the risen loaf, which promptly subsided to half the size. I baked it and now I have another paving brick. What can I do to prevent this happening again.
Next time you make this bread slash the loaf before the final proof.  You degassed the loaf when it fell the last time.