It is rather unprecedented, but it is also illuminating. Two possibilities seem likely at this point (in no particular order):
1. The Obama "phenomenon", manufactured though it is, indicates that there are many Democrats who really do NOT want to see Hillary Clinton's name on that ballot; and/or
2. Hillary and her people are actually behind Obama's "out of nowhere" appearance in these earliest days of the campaign. He's Hillary's "stalking horse".
The old "bait and switch"? Wow, hey it could happen.