jremington:
It sounds like the laptop is powering the Arduino, so use a separate supply -- batteries or wall wart.
I get the same behaviour when using a 9v battery to power the arduino unfortunately 
(no serial connected). This is initially how I discovered it doesn’t work as intended, because I wasn’t receiving any IMU data in MATLAB when I wasn’t able to open the com port.
Edit: I will provide a video if required demonstrating the behaviour.