I have said for a long time, ever since summer 2015, that I didn't think Hillary would be the nominee. They will let her take all the slings and arrows and then Biden or Warren or both will swoop in and take over, reducing the time available to damage them. Especially if her poll numbers are low, which they are. The leverage for that is the email scandal. Obama will promise a pardon on his way out, and prevent prosecution. They can also blame it on health reasons.
Point being, Warren is upset because she believes she may be on the ticket this fall, either as candidate or as Biden's VP.