Some progressives are pissed at Warren for a few things. One, not endorsing Sanders in the primary (and worse, not having the guts to endorse Clinton before the whole process was over and it was obvious Clinton would be the nominee), which they blame for costing Sanders a win in Massachusetts. Two, her speech at the DNC in which she said that Clinton believed and would do a bunch of things that said progressives were convinced she didn't and wouldn't. Three, her utter silence on pipelines like DAPL (especially that one considering her claimed Native American ancestry). Four, the reports that she's looking to get onto various armed services and foreign policy-related committees, which might make her more hawkish.
They might not vote for her opponent, but if enough of them just skip that line on the ballot (or go third-party), it could cost her re-election.