Both wipers are connected to a single motor that uses a set of mechanical linkages to move together. My best guess is that either the motor is failing and doesn't have enough torque to move the wiper blade assembly past the vertical position or the linkage assembly itself is getting rusty and is causing to much resistance for the motor to overcome. You might just be able to get some lubricant in the assembly and work it loose. Take a look at this video to see what I'm talking about.