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.

In medicine there are clearly defined rules for authorship, although they are not generally followed. Computer science does not even have such rules, unfortunately. The International Committee of Medical Journal Editors has an excellen page about this at http://www.icmje.org/recommendations/browse/roles-and-responsibilities/defining-the-role-of-authors-and-contributors.html

• Substantial contributions to the conception or design of the work; or the acquisition, analysis, or interpretation of data for the work; AND
• Drafting the work or revising it critically for important intellectual content; AND
• Final approval of the version to be published; AND
• Agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

(notice the conjunctions!)

Additionally, anyone who contributed substantially must be listed as author.

I strongly suggest that you ask your professor why they want to be removed as author. Communication needs to be strong between authors!

I know that I once added my professor as a courtesy, and he insisted (rightly so) that his name be removed, as he was not a substantial contributor. I needed to learn to stand on my own academic feet, he said. "Honorary" authors should be avoided at all costs, as they indeed did not contribute to the work or stand behind it.

Even though the professor thinks the paper is low quality, that does not mean it is low quality for you too.

A professor might not want to appear in a boarderline paper, and that is very natural. On the other hand, any paper significant enough to get published in a peer reviewed journal or proceedings (except fraud ones) is valuable for a master's student.

The number of possible reasons for him to withdraw from authorship is maybe hundreds, if not thousands. But the real question is, is it a reason for you to withdraw as well?
I think not.