In general, in aircraft with tandem seating, the rear seat (having the Radar Officer) ejects first, followed by the forward (pilot) seat, after a delay of ~0.3 seconds. This is done for a few reasons:
If the pilot seat is ejected first (or both are ejected simultaneously), there is a possibility that the pilot seat may collide (as it will be dragged backwards due to wind force) with the copilot seat or damage the (rear) canopy during ejection.
In some aircraft, the pilot can eject only after the rear seat is ejected. This is so that, in case the rear seat fails to eject, the pilot can still control the aircraft and 'pop out' the Radar officer's seat by maneuvering. For example, the F-4 procedure called for the pilot to roll the aircraft inverted with a positive 'g' and then pop the radar officer with a negative 'g'.
In some cases like the (Mig-15 UTI), the rear seats were ejected first simply because the gas jets from the pilot seat ejection mechanism made ejecting from the rear compartment impossible.
I'd think the main reason would be to avoid cooking the rear seat crew while the rockets were firing as well as to avoid an accidental collision between the front seat and the rear seat crew. After all - the jet is likely to be travelling forward at a relatively high velocity when the two are coming out of the plane (or what's left of it).