Benchmark is desperate to get out. Why do you think that is? Do you think they'd be all hands on deck trying to get out of a rocket ship investment? Of course not.
Uber's unit economics are garbage, and will remain garbage until self-driving technology becomes universal. That will not happen in a timeframe that Benchmark is comfortable with - or to put it another way, within the preferred close dates of their fund - and even if it did, they clearly have little confidence that the current leadership (or lack thereof) are the right operators, either now or in the future.
The ship is sinking. They want out. Softbank wants in - for reasons I can't understand other than they have the money and want to spray it around aimlessly - and this bit of messiness is surely throwing a wrench into the plans.