This question relates to the movie Wall Street: Money Never Sleeps. Why does the rumor of nationalisation affect the interest in Hydra Offshore shares?
When something gets Nationalized, it becomes a government entity. It is no longer a private business designed to make money for shareholders. Additionally, the former owners may or may not be paid for the company, similar to what we have in the U.S. called "Eminent Domain" for property. Although, with Eminent Domain the owner must be paid a fair price for their property, with nationalization of a corporation the shareholders may lose their entire investment.
Once the rumor was spread by Jake, the shares begin to dive as people begin selling their shares before they're left with nothing.