Being a former flour miller it sounds like you are suffering from a problem commonly known as flying top crust.
This can be caused by some of the following reasons.
Dough is too tight Too short of proving period Over treatment with improvers.
If you have recently started using a new bag of flour it could be a problem with that caused by wrong settings on the Ascorbic Acid addition or it could be down to the Alpha Amylase activity in the flour.
This is a natural occurring constituent in the product but it can be altered by powder addition by the miller.
What type of flour are you currently using?
I would expect our forum poster BT will add a comment to this thread in the near future as he to worked in the flour milling industry and will give you a fuller explanation as he worked in QA within the milling industry.