1
general::
When something occurs it often does so to excess. For example, First Aunt Sue said she and Uncle Harry were coming for the weekend and then my sister and her children said they were coming too?it never rains but it pours. This expression may have come from either a book by Queen Anne's physician, John Arbuthnot, or an article by Jonathan Swift, both entitled It Cannot Rain But It Pours and both published in 1726.
American Heritage Idioms
2
general::
Phrase(s): It never rains but it pours.
Prov. Good (or bad) things do not just happen a few at a time, but in large numbers all at once. • Fred: I can’t believe this. This morning I had a flat tire. When I went to the garage to get the tire patched, I discovered I didn’t have any money, and I couldn’t even charge it because my credit card’s expired. Jane: It never rains but it pours.
McGrawhill's American Idioms And Phrasal Verbs