Some fact not otherwise known is being hidden from the public. It may have a bearing on the issue of his Constitutional eligibility to serve as President, or it may not. In either case, he is so concerned about its possible public impact that he would rather damage the perceived authority of the Constitution (a direct violation of his sworn duty as a public official of the United States) than let it be known.
It's hard to imagine anything contained in a birth certificate that would be matter for such personal embarrassment as to justify this dereliction of public duty. No sane person, and I think especially no right thinking American, would hold an infant responsible for the sins or views of his parents. But perhaps merely avoiding personal embarrassment is not the reason for his efforts to conceal.
Obama is probably a citizen, just not a "natural born citizen," since his reputed father was not a citizen, nor even a naturalized citizen. BHO, Sr. was merely, one hopes, a "legal resident alien."
I believe what he is hiding is the "Barry Soetoro" story. He may well have registered for college under that name, and claimed foreign status in order to qualify for some sort of benefit. That fraud alone would not wipe away his citizenship, nor would alter his fundamental lack of "Natural born Citizenship" but it sure would be embarrassing and illegal ... perhaps enough to have him disbarred if nothing else.
He even hid that "Barry Soetoro" identity from the Illinois Bar, upon applying for admission. Dr. Orly Taitz is now claiming that both POTUS and his mother have used different SS#s at different times. IF TRUE, we definitely have a whole 'nother problem.
So beyond bringing this up interminably and at boring length, what the Hell are we supposed to do about it? One avenue is to have as many state legislatures as possible pass laws requiring that state electoral boards certify presidential candidates' eligibility status in every regard, prior to giving any candidate the state's electoral votes.
I believe a drive in this direction will bear more fruit than the endless speculation about documentation POTUS DOES NOT HAVE TO PRESENT.