It generally means an attempt to conceal or hide something. For example if you got a tattoo that you regret. You could get a cover-up tattoo that hides the first one. It is used a lot when referring to government and police agencies. In this context it would likely mean that someone - whether a police or government agency tried to hide or conceal the "big escape in portland".

Cover-up is a phrasal verb that means to attempt to hide information or evidence because it may show that someone has broken the law or done something wrong, In your example, this could mean that the police were involved or knew about prisoners escaping from prison and tried to hide the evidence because it was embarrassing.