Doc left to grab the separation agreement shares. It was Uber beneficial for him to do so. Chances are with these employees it would have been much less beneficial to do so.
I just spoke with a high level HR person in New York. She said that retention bonuses 95% of the time are used for upper level management, who are, because of their roles, privy to the fact that they could potentially be out of a job shortly due to M&A.