Some thought the direction was wrong - not gathering enough test data (unlike Tesla).
Some thought they didn't have a big enough slice of the pie, and by moving to a startup and replicating the work they could earn way more through an acquisition by a carmaker.
Some saw the tech as fairly old. Remember much of Waymo's code was written by Urmson 10 years ago. Probably all C/C++. Not very nimble and quick to extend.
Some saw the fundamental approach wrong. Why not use neural nets for path planning and decision making rather than hand coded rules which will never cover enough cases to be good enough?
Some didn't want to be part of Googles risky approach. Too much pressure to get the cars on the road could kill people, and killing people, or the threat of it, is rather bad for morale. The leaders had this "even if it isn't perfect, it's better than current drivers" approach. Thats little comfort if it was your off by one error which caused the car to plough into a group of schoolchildren.