Right now SEIU (union of service employees) has to negotiate for health care coverage for their members in each contract and shop, against employers who are trying to save money on health care as the cost of health care rises each year. Obamacare would change this. I imagine most of the union members would like to be sure that they would continue to have health care coverage, and not have to fight for it each time a contract is renewed. Many union members would have nonunion relatives, friends or neighbors for whom paying for health care is a constant worry. Since they are brainwashed by the union into having bleeding heart liberal attitudes, they would support Obamacare because it would benefit those people even though they did not need it for themselves.
So maybe SEIU is working for Obamacare because its members want it to.