Occasionally a heavy tap can 'flex' on some large stainless steel sinks. This can be rectified by your tap being braced, and it is not unusual for people to have this kind of problem with a stainless steel sink, and is not a problem emanating from the sink or tap being defective in any way.
To brace the tap, you either need a piece of wood or chipboard which can be screwed across the underside of the worktop cutout and the existing tap can be fixed through a hole in this, or you can utilise a tap brace. In this way the tap is effectively secured to the underside of the strut and not to the underside of the sink, and this reduces any tendency for the sink to flex when the tap is turned on or off.
Franke have recently produced a new patented Tap Brace which will provide a solution to the problem of 'flexing' of heavy taps on some stainless steel sink models. Fitting directly to the worktop, the tap brace has a variable collar which fits all models of taps.