The NLRB has officially filed a complaint against Tesla for anti-labor tweets made by owner Elon Musk in May. The tweet threatened that workers would lose their stock options if they unionized. Shortly after the complaint was filed, Musk tried to explain his reasoning behind the comment, saying that the union was unlikely to let employees hold stock in Tesla. Nevertheless, the Board found Musk’s comment threatening and illegal. Musks social media activities have also sparked multiple lawsuits and an investigation by the Securities and Exchange Commission.
See "Elon Musk on Twitter is now a labor issue", Kate Gibson, CBS Money Watch, August 29, 2018