A Tesla spokesperson told Bloomberg that the two are not – as others have hinted – leaving because of problems with the Model X, especially the falcon-wing doors. The spokesperson said, "This is not about the Model X. After being at Tesla for over five years and leading its production team for the past three years, Greg Reichow has announced his intention to take a leave of absence from Tesla so that he can have a well-earned break."
It was all hearts and smiles in a joint email that Tesla CEO Elon Musk and Reichow sent to Bloomberg: "Greg and the team deserve a lot of credit for building an all-new manufacturing organization from the ground up and for making Model S and Model X a reality. We're confident that with the strength of the team, high-quality manufacturing at Tesla will continue," Musk wrote, while Reichow continued: "My belief in Tesla's ability to successfully deliver great cars and inspire the world to drive electric remains as strong as ever." There's a good list of other executives who have come and gone at Tesla recently here.
Tesla hasn't said when Model 3 production will start, but has promised the first deliveries in late 2017. We will see if the executive departures or any possible change in the Model 3 timeline will be addressed in the company's earnings call, scheduled for a few hours from now.