Game of Chicken The mainstream view seems to be that Tsipras thought the opposition would cave at the 11th hour. When they didn't, he did, because he wasn't prepared to actually exit the Euro. Subscribers to this view believe that Tsirpris is simply an inept, amateur politician who really had no plan for the negotiations.
Lunatic Fringe Some elements of the lunatic fringe think that Tsipris was personally threatened and it was made clear that he would be murdered à la Litvinenko if he didn't sign. The same lunatic fringe also says that Russia would bail out Greece in order to undermine Europe, which Russia is clearly in no position to do. In any case Russia is a kleptocracy which would loot Greece far worse than anything Germany and Finland would do and surely the Greeks know it.
Buying Time Perhaps the Greeks decided that more time was needed to arrange an orderly exit on their own terms, so they are pretending to go along while secretly building the Drachma printing presses in the basement.
Any other ideas? Email me.