What is the cause of the crust lifting from the crumb of my last bread loaf?
Not really sure, based on your description, but what I've experinenced in the past is a top crust "lifting" off of the crumb basically a large hole just below the top crust. I think if you search the site you will find some photos of this effect.
Generally this is caused by too little surface tension during shaping, and slashing that is not deep enough (to release the steam created during baking I think).
I'm sure someone with a better answer will weigh in here, and perhaps has a link.
I have this problem rear it's ugly head from time to time. I recently baked two identical loaves, One had it one did not? I have heard lots of reasons. The most consistant is improper shaping. I can't tell you how many ciabattas I thought were money that just turned out to be a giant top bubble. I would love to know the answer to this.
Da Crumb Bum