Surprise announcements can trigger a chain of events that affect the stock price of a particular company, competitors and other companies that might be suppliers to or customers of the primary company.
Announcements of Mergers and acquisitions (M&A) are a frequent type of surprise announcements. Mergers and acquisition is a category of announcement that deals with corporate structure. When the announcement is publicly made, analysts use that information to factor into the company's long run market outlook. A merger or an acquisition could signal to an analyst that one particular company is financially weak, and it could downgrade its long run outlook for that company. Alternatively, an M&A announcement could also signal to analysts that a company has a chance to increase its market share and henceforth its profits. These announcements typically make the biggest splash in a top-heavy industry, such as automobiles, pharmaceuticals, or healthcare, with a few strong players dominating a market that is researching, developing, and innovating new products.
Other announcements that can have a variety of impacts on asset valuation can come in the form of a major court decision, news of development of new research, and innovation, FDA approval of a new drug or a new cheap source of raw materials. Any of this news has the potential to impact a particular company and, in some cases, competition, suppliers, and customers of that company. In many cases, there will be signals that analysts pick up on ahead of time. Investors who are actively making investment decisions based on a particular company or within a particular industry need to be aware of how a surprise announcement might affect their investment.
Fixed-income market participants also make forecasts on the long-run health of a company to project the likelihood an issuer of debt might default. A company's instruments are subject to move in the event of an aforementioned announcement or if a private rating agency (Standard and Poors or Moody's) changes its outlook on their debt. If a news announcement is seen as positive for a company, its fixed-income instruments might seem more creditworthy, its bonds will rise in price, and its yield will decline. The inverse is also true.
Take a look at the creditworthiness ratings of different countries by Standard and Poors.
World countries Standard & Poor's ratings
An example of the credit ratings prescribed by Standard & Poor's as a result of their respective long-term liability analysis for debt issued at the national government level. Countries issue debt to build national infrastructure. Look how expensive it is to raise capital for such projects based on geographic region.