Volkswagen AG’s dramatic move to oust its combative chief executive officer was set in motion a week ago, when his backing from the billionaire Porsche and Piech family began to crumble.

Unwavering support from the reclusive clan that majority-owns VW had helped Herbert Diess survive frequent clashes with powerful worker representatives. But as key project failures combined with worker discontent, the family concluded he had to go.

The decisive day was July 20, according to people familiar with the deliberations. The top committee of VW’s supervisory board, comprising family representatives, officials from the German state of Lower Saxony officials and labor leaders, determined Diess’s time was up. He learned this around lunchtime the next day, still jet-lagged from a visit to the automaker’s SUV factory in Chattanooga, Tennessee.