What causes the 'knee' in higher order filter magnitude response?
Forget Sallen/Key for the moment, go back to the equivalent passive implementation of the same filter. (e.g. search "passive butterworth filter" for examples.) You'll see it involves an L and a C per 2nd-order filter section, thus it's basically an L-C resonant circuit, with resistive damping to control the resonance (the height of the knee). 

Having understood this, then you can use a C in the feedback path of an amplifier to simulate the L in an L-C resonant circuit - and the Sallen-Key circuit is basically one example of this. As "FakeMoustache" comments, the gyrator is another.