Possible reasons for an author to withdraw authorship from an almost ready paper
In general coauthors should communicate with each other about what they're doing and why.  It is strange to withdraw as an author for "personal reasons," as this is a purely professional decision.  

A supervisor of a graduate student has much more responsibility: they have a mentoring role on the project and they are training the student both intellectually and professionally.  In my opinion it is not acceptable for an advisor to take their name off a paper without offering any real explanation.

Of course this puts you in an awkward position.  I would nevertheless recommend trying to talk to your supervisor about this, preferably in person. If you have your own concerns regarding the quality of the paper, then I'm sorry to say that you should think very carefully before going ahead with the submission.  It might be appropriate to tell your supervisor that you will delay submission until after discussing the reasons for their withdrawal, but I leave that decision up to you.