If this is true, it sounds like somebody didn't properly perform their due diligence before signing their options agreement. Although it's never right for a company or investor to exercise this buy back when it comes to an honest, hard-working employee, the onus really falls on the employee ensuring that this clause never sees the light of day in their contract in the first place. Perhaps in the event of "cause", one could make a case, but certainly under no other condition.
EDIT: It's an unethical clause to begin with - absolutely agree with the comments. Just saying that you can't count on anyone besides yourself to act on behalf of your own best interests.