A problem that exists in all oscillators is that it is nearly impossible to get an exact amplification of the signal. If the amplification is too small then the oscillator signal will decay to nothing, however if it is too large the signal will keep on amplifying until it hits the rails of the op amps. This means that some sort of nonlinear feedback must be implemented with these oscillators so that the signal provided will actually be a stable sine wave.
The bubba oscillator (as well as other phase shift oscillators) solves this problem by the very nature of the op amps, when the signal is amplified back into the circuit the signal gets clipped at the peaks of the sine wave. This is because the amplitude is reaching the rails of the op amp allowing the signal to stabilize and providing the nonlinear feedback needed.