It's not just raining, it's pouring rain - heavily! So a (bad) situation does not simply happen, it happens in excess.

In 1972, there was a song that became a big hit in the U.S. -- "It Never Rains In California". A poor guy is singing about his failure to become a star in Hollywood. The lyrics speak to the idiom you are asking about: ~Ooh, that talk of opportunities, TV breaks and movies, ~Rang true, sure rang true ... ~I'm out of work, I'm out of my head, ~Out of self respect, I'm out of bread [money], ~I'm underloved, I'm underfed, I wanna go home ... ~It never rains in California, but girl, don't they warn ya? ~It pours ... man, it pours! The origin of "It never rains, but it pours" is unknown, but its use goes back at least to the early 1700s. So it is a time worn proverb that is still around today. The use of the phrase in this song is ironic because in "sunny" southern California there is very little rainfall. And its meaning, as the lyrics suggest, is simply that, so often, bad things do not just happen a few at a time, but in bunches, all at once. Certainly, you have to think that somewhere, at some time, in a place suffering from devastating cycles of droughts and floods, this sad-but-true old saying was coined. (Note: You can catch that great old hit song on YouTube!)

Most people say "When it rains, it pours" instead of "It never rains but it pours". It just is a phrase illustrating that difficult situations rarely happen in isolation. In case you didn't know, "It is pouring down rain" means that it is raining really hard.