Overall, the occurrence of pack journalism is largely due to reporters' reliance on one another for news tips and use of one single source for their information (which could often even be the very subject they're covering).
[1] Modern pack journalism practices no longer require a physical proximity of campaign buses or shared press rooms.
This, in turn, leads to numerous news organizations highlighting and/or publishing similar or even identical stories.
A significant short term consequence of pack journalism is that it turns minor news stories into national headlines.
Watching and reporting on one person over and over provides viewers with one perspective on the race and leads to a shortsighted view of the campaign overall.
Conformity within the news has been related to "agenda setting", which is the result of the press's influence over audiences in conveying to the people which events are important simply by covering them.
In the United States, when a reporter breaks an important story to the media, other news organizations pick it up and spread it further.