Layoffs do sometimes happen this way. I was an EM at a company with layoffs where line managers were not told at all about layoffs or included in deciding who to lay off: all discussions happened at the director level and up.
I know it's true this happens because in a recent round of layoffs, my manager not only found on the very same day, but got fired himself.
During mass layoffs, your immediate manager is often not told in advance in order to stop leaks and also because he/she may be one of the people laid off. (You cannot tell only some managers and leave others out, because managers of the same level talk among themselves. The ones left out would know why).
Ha! And, you believed that? They were "told" not to "tell" about it to anyone until official announcement.