Disclaimer: It's been a while since I flew the 320 and I don't have much time on it to begin with.
The PTU will come on with a pressure differential of more than 500psi between the Green and Yellow systems. The external cargo door switch works by remotely running the Yellow Electric Pump to pressurize the Yellow System, while the Green stays unpressurized. If the PTU wasn't inhibited it would start running as soon as the cargo doors were operating as it creates a pressure differential between the two. 40 seconds seems like a reasonable time for the Yellow hydraulic pressure to bleed back down to below 500 PSI so the PTU logic doesn't sense a differential.
If only IAI had thought of a similar logic on the Westwind fuel pumps so you don't have to play the MAIN->ALTN->MAIN game during engine start.
