Danielmc on the BM Bikes & BM Riders Club Forum provides this insight. I always knew our bikes of all airheads didn't deserve the derogatory version.
The term Gummikuh (rubber cow), originally ascribed by the motoring journalist Ernst Leverkus to the earles-forked aircooled BMW twins, referred to those machines' habits of rocking between their soft front and rear suspension and the rising of the rear-end under power: cows stand up rear-end first. In Germany modern BMWs are still referred to occasionally (and affectionately) as Kuh whilst the smaller 'bikes can be referred to as Kälbchen or calves.
Calling them cows was not meant by Leverkus to imply they possessed evil or dangerous handling, and similarly the modern German usage is not derogatory.