One angle to consider is the shark attack factor.
That's when there is an event that happens with a certain degree of regularity...such as 3-5 people being attacked by sharks every summer. It is normally very lightly reported by the national news media if it is reported as all. State media covers it a bit and of course local media as well.
Then something happens to make it more newsworthy. Maybe a famous person gets attacked. Maybe a movie on the subject gets made. Whatever the reason, the national media latches on one story and it gets a lot of coverage.
Then when the next shark attack happens even though it actually falls in the norm, it gets a ton of media coverage. As does the next, and as does the next. The average viewer never knowing that 3-5 shark attacks per year is average, instead just sees a chain of high coverage events and may well conclude 'boy swimming in the ocean sure is dangerous!' and 'why are all these shark attacks happening all of a sudden'.
Thinking of that filter, how many bomb threats do you think the USA as a whole has happen every year? Is this actually unusual, or does it just SEEM unusual due to increased coverage in the media?
Is there some reason why there would be increased media coverage of bomb threats? I think that people being more afraid and seeing potential terrorists around every corner really helps the NSA, PRISM, etc.