It's just a case of trying to make it sound less serious. No foolin'. Here's how it happened.
Back in the 19th century, there were a couple of big financial panics- one occurred in the late 1870's, I think. Sometime later- probably in, oh, about 1929
- someone said- "You know, "panic" sounds bad. Why, people might even panic! Let's call it a "depression"- that sounds so much less serious."
And, Shazaam! Rather than the Great Panic, like those of the previous century, we had the Great Depression of the 1930's.
Sometime later still, someone said,"You know, the word "depression" didn't work out so well- the 1930's turned out to be a real *****! The next time the economy slows down a lot, let's not call it a depression- after all, people might panic! Let's call it- oh, I think "recession" would be nice. Yes, let's call it a recession."
And so they did. Since then we've had several recessions, but none as bad as the 1930's- so far!
Not sure what the next less serious sounding term will be- but you can be sure that eventually, "they" will come up with one.
Readers still curious about the practice of changing events, by simply changing the event's name, are directed to "Hitchhiker's Guide to the Galaxy", by Douglas Adams. There, they don't really call destruction of the planet anything- they just keep saying, "Don't Panic!"