Why should the phase of a PSK signal be changed at the zero crossing of the RF signal?
Why Any abrupt change in a waveform creates high-frequency components, and the more abrupt the discontinuity the wider the resulting spectrum. If a PSK transmitter flips phase while the RF waveform is at a voltage peak, the signal jumps instantly from a large positive value to a large negative one, producing sidebands far from the carrier. Switching at the moment the waveform passes through zero makes the transition continuous in amplitude, so the occupied bandwidth stays narrow and the signal does not splatter onto neighboring frequencies.
Watch out Carrier suppression is a property of the balanced modulator or the chosen emission type, not of the timing of the phase reversals, and zero-crossing timing actually adds complexity to the modulator rather than simplifying it, so the all-of-these choice fails too.
Flip phase where the waveform is already at zero: no voltage step, no splatter, narrow bandwidth.