Not treason, not even a crime. Obviously anyone participating can be fired, including union members since they're not striking over workplace issues. That said, she's a high profile Democrat, advocating for the Democratic Socialists of America, a clearly socialist political party. IMO Democrats from Pelosi/Schumer on down own AOC and The DSA, and their views.
On their website they ask the question, Bernie's out, now what? Clearly AOC has an answer.