It's unclear if Gurley actually wants to leave the board. Part of me thinks that he knows something really bad is coming down the pipes (e.g. potential criminal liability re: Otto, etc) and he wants to distance himself now. It's half a conspiracy theory, but makes you think ...
It's got to be the Otto lawsuit, that's obviously coming, after Levandowski dropped the ball with that "plead the Fifth" stuff.
Even the Google network forensics team have him bang to rights[1] taking data while on-site at the campus, Google would be mental to not launch a volley and destroy a potential competitor in the process.
And you don't drop $680m on an acquisition without going through board approval first, surely? Somebody's definitely complicit.
This was the same thing that popped my mind when I read this news.
Also: if he leaves before something really bad happens to the company, he might claim he left because the unethical behaviour of the company didn't align with his ideals, perhaps not hurting his future prospects at the boards of other companies.