I'm not sure of the cause. I asked about this problem and there are a few issues (some related to HHH-5024).
From a recent IRC discussion about this:
Part of the problem is that there are just lots of incorrect mappings that crop up there. People trying to override mapping information incorrectly. More defensive checking is really what is needed there.
I suspect that it might be easier for you to help contribute a Hibernate patch/fix to add the defensive error checking.