The growth of oscillation in a quadrature oscillator that employs phase shifters in the coupling-path between a pair of LC-loaded negative resistance cores, is analyzed. Such an oscillator is known to have two stable modes of oscillation. Under a noise-initiated startup from an unstable initial condition, quasiharmonic assumption and the Method of First Approximation are used to demonstrate that compression mechanisms lead to preferential enhancement of one mode, while attenuating the other. The magnitude of phase shift in the coupling-path is shown to directly affect the ratio between temporal rates of mode buildup and decay.