Why all the Starbucks unions?

This article has an answer:

"The massive Starbucks unionization drive is about striking a blow against the authoritarian power of management. [...] Starbucks founder Howard Schultz [...] is the classic example of the proprietary capitalist who sees any threat to his authority as a personal affront."

So what is Schultz doing to fight back? He's going to raise wages and improve benefits,* things the unions are fighting for, but only for employees that work in non-union stores! This is in direct violation of NLRB law. Also see the leaked video of him instructing store managers to union-bust. 

Bottom line is that workers want some negotiating power and Schultz doesn't want to give them any whatsoever. TFB. Workers are exercising their right to unionize and taking back that power.

* Of course there's a catch to the aforementioned benefits. They are only available if the employee works at least 20 hours per week. And managers are instructed to rigorously manipulate schedules to prevent most workers from getting the necessary hours.