Did the Panamera just pop out of thin air?
Far from it.
Porsche has been flirting with a four-door sedan for ages -- going back to the first Auto Union P52 designs by Ferdinand Porsche, in fact, that had a central driving position and twin seats beside him.
In later studies, the rear-engine design would be the biggest difficulty in bringing a four-door sedan to market.
So when the 928 and its front-engine setup was present, the idea seemed like a natural fit.
In fact, the 1988 Panamera concept that came a year later was a strong hint at future 911 993 generation design ideas as well as laying the overall template for a 928 re-style that was expected, but never came, in the early 1990s.