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STORY: Shares of Starbucks jumped on Wednesday, after the coffee chain announced that its longtime former CEO Howard Schultz will return temporarily.

It comes as Kevin Johnson, who's been CEO for the last five years, said on Wednesday that he will retire from the role on April 4, as a unionization drive at U.S. cafes heats up.

Under Johnson's watch, the company put more emphasis on to-go and drive-thru orders.

Customers also placed more orders through the Starbucks mobile app, which has led to long lines and overworked employees in some areas.

That barista burnout contributed to a surge of union organizing at U.S. locations.

On Tuesday, a federal labor board accused Starbucks of unlawfully retaliating against employees in Phoenix, Arizona, for trying to unionize their store.

Schultz has long said Starbucks did not need unions because it works so closely with employees.

And he's already had a run-in with Workers United, the union now representing employees at six stores.

He has also faced criticism for past remarks on the issue, including likening the company's pro-employee stance to Holocaust prisoners in concentration camps sharing blankets, according to media reports, which sparked a backlash on social media.

The board of directors at Starbucks is expected to select a new leader by the fall.