People always like to play pranks and like to enjoy seeing others panic. This is very common behavior within every human. Many people don’t let it come out as they know how to manage their social behavior and dignity. But some people never learn any thing good in whole of their lifetime.
Since it is not possible to play tricks directly they use some medium like emails or phone calls or even letters. There are many people out there always looking for a chance to create a hoax, because there are really a big number of people easily vulnerable to such things. It is them who encourage such ill minded psychos to create more such things.
So before you panic to anything, just try to figure it out whether they are hoax or jokes. All it requires is some commonsense and good idea on the topic. If you don’t have idea on the topic, use Internet because there is much useful information out there which you can access easily.