By setting up its own rules for ratification, the Constitution was breaking the law. The Articles of Confederation had stated that any changes to it had to be unanimously approved by the states. The Framers of the Constitution were not supposed to be writing a new Constitution. They were simply supposed to be amending the Articles. Instead, they wrote a whole new Constitution and set up new rules for ratifying it.
The rules they set up can be found in Article VII. They state that the Constitution would go into effect as soon as it was ratified by conventions in nine states.