The saying "when it rains, it pours" is an idiom that means when bad things start to happen, many other bad things tend to follow in quick succession. It suggests that misfortunes rarely come alone. While often used for negative events, it can also describe a sudden abundance of good things.
The phrase gained widespread popularity due to an advertising campaign by the Morton Salt Company in 1914. They used the slogan to highlight that their salt, with an anti-caking agent, would flow freely even in humid conditions, emphasizing that "Morton Salt would flow freely even in humid damp or conditions". This was a clever twist on an older English proverb, "It never rains but it pours," which had a more negative connotation.