Failed Philadelphia Convetion
Have the 1787 convention breakdown over something...slavery is the most likely.
When everybody returns for round two in 1788 or 1789, have the division of government be part of the compromise package, with each section getting something and a small state in each section gets a piece as a bribe/making sure that one of the three large states don't get too powerful.
For something on Slavery, Mary land gets the seat of the executive.
The Middle states, New Jersey, gets the legislature and
New England, maybe Connecticut or Rhode Island for giggles, gets the judicary, because nobody thinks it's all that important.
David