So, looks like a lover's quarrel between Sanders and Warren. Evidently, he had planned to bring her on as his VP … until AOC came along. He dumped her for a younger woman.
https://www.msn.com/en-us/news/elections-2020/the-sanders-campaign-looked-into-whether-warren-could-serve-as-vice-president-and-treasury-secretary-at-the-same-time-according-to-report/ar-BBZ5yRu?ocid=spartanntp