In addition to the answers above, I feel like plenty of Democrats moved to the left following Trump's election
It's this. There's nothing like electing Donald Trump and watching him bumble his way into partially dismantling the ACA, threatening to destroy Medicaid, banning immigrants from multiple countries for no justifiable reason, and excusing white supremacist violence which makes you pretty open to voting for the most vocally anti-Trump person in the room, especially if that person is contrasting him/herself by embracing the DSA in a safe blue seat.