Investigating the cause of the drift for this system
Mechanical coupling means some mechanical connection between parts of the system. In this case the trouble-shooting guide is advising you to check for mechanical coupling or connection where there shouldn't be any - especially "between the tool-plate and the sensor body". This would be equivalent to having enough debris inside your bathroom weigh scales to affect the reading in certain situations - maybe above a certain weight.

Since the device is designed to be sensitive to wind I would shield it, monitor the excitation voltage, temperature and any other variables you can think of while logging the output. Check for mechanical stability, debris, loose mechanical connections, etc.

If all of that looks OK then you may have an electronic failure. If you're going to open it up have a look for leaky or bulging electrolytic capacitors.