Because the MBP has it's own sensor and when both are active it causes problems.
There is a version of this drive (ST9500420ASG ) that does not have the sensor. ZipZoomFly carries it. Interestingly, my brand new MBP (a CTO received 8/19) has the G version of the drive.
I don't believe the drive has any interface to alter the sensor, its on and will cannot be changed. That means you're only recourse is to turn off the apple sensor, there by allowing at least one motion sensor to work correctly.