Trevor Milton's stepping down as Nikola Corp.'s /zigman2/quotes/208704275/composite NKLA +4.30% chairman was "shocking" and will leave investors with some "white knuckles," said Wedbush analyst Dan Ives, as it is now all about execution with the partnership with General Motors Co. /zigman2/quotes/205226835/composite GM -1.66% a "linchpin to its success." Nikola's stock plummeted 36% in premarket trading, putting them on track to open at a four-month low. "Trevor stepping down voluntarily at Nikola will be perceived by the Street as a major near-term gut punch for the company's lofty EV ambitions as he plays a key role strategically in driving the company's vision," Ives wrote in a note to clients. "In a nutshell, Nikola is still a 'prove me' stock, which speaks to why we are watching this from the sidelines with a neutral rating until we can get more confidence in the EV and hydrogen fuel cell execution story over the next 12 to 18 months." Milton stepping down comes a little over a week after short-seller Hindenburg Research said Nikola misled investors.