Report claims Nintendo "forced the company's hand"
www.nintendolife.com
According to Kotaku, the individual known simply as 'Mike', was a GameStop employee who shared a photo of the company's inventory database after the initial leak in December - speculating the system would be revealed ahead of Nintendo's new gameplay trailer for TotK.
Roughly two weeks after the showcase, Mike was fired on 11th April. Where it gets interesting are claims Nintendo traced the leak back to Mike via his Reddit and associated social media accounts and "forced the company's hand, demanding the employee be terminated over the leak." Another GameStop employee reportedly corroborated Mike's account to Kotaku.
Mike apparently didn't think he would get in trouble, so he "wasn't really" trying to cover his tracks along the way. During the phone call of his termination, Mike's manager told him "this is your favorite company and now they hate you."