Another reading is this: CEOs don't quit without notice [1], it's bad form and bad for both the company and for the reputation of the person leaving. Regardless of the value he as an individual was providing it speaks to substantial disfunction in the overall leadership of the company and a major lack of confidence from the person who presumably had the best idea of how the company was doing on the whole. The market suddenly learning those things resulted in a substantial market correction. It doesn't really matter whether he was a particularly good or bad CEO; the situation would indicate something is majorly wrong in either case.

1: Obviously exceptions will exist for unexpected major life events, etc.