My uninformed guess is that it took both pilots roughly the same amount of time to run through their OODA (Observe, Orient, Decide, Act) loop and conclude their plane was not recoverable and eject.
And for both crew members of the same plane ejecting at the same time, I think the ejection of the second personnel is automated should the first one eject. (Not familiar with the F-18 fighter at all but I know it's like that in other fighters with two crew members).
"What is the Boyd Theory? To many it is simply the OODA loop depicting the human behavioral cycle of decision-making. To others it is a description of command and control. To true believers, it is a profound theory of warfare."
-- Some other Military Dude
By the timing seen in the video, the front seater -- the pilot -- ejected in both jets at close to the same time. That automatically ejects the electronics officer first then the pilot momentarily after, as otherwise the front-seat ejection would wash the rear-seater with the launch exhaust from the front seat ejection.
If it looses either the lift or main engine the resulting pitch change would be too rapid for a human to react to so the system triggers ejection.