Actually 2 ships left from England, the Mayflower and the Speedwell. The Speedwell had to be repaired for leaks on the way from The Netherlands. On the way to America the Speedwell had to pull into port once again to repair the ship for leaks. Concern mounted that the Speedwell wasn't going to be fit for the journey so a decision was made to leave the Speedwell behind and to have the Mayflower be the only ship to make the trip. And that meant some people decided to return to the Netherlands rather than set sail on an over crowded Mayflower.
Considering how long the passengers had lived on the ship for prior to the departure from England and then the subsequent trip, and then exploring the area off of the Cape and NY the last thing anyone wanted to do was get back on the Mayflower. And a return to England would have also meant a return trip to The Netherlands for many of the passengers.