accelerometer interference
You may well have induced-electrical-noise issues.   

Test: 


Place the PCB and cable on a flat surface and 


Weight the cable with a cup or similar non conductor near the PCB. 
This allows you to move the cable slightly when you touch it without moving the PCB.   
Now touch the cable on the non-PCB side of the weight.   



If you get similar results it's probably electrical noise.
If so, you need to examine the circuitry where the USB cable interfaces to see if it may be allowing induced signal to interfere with the accelerometer signal.