Why are ballistic missile submarines called boomers?

The term comes from the "B" and "M" letters in the acronym, "FBM", which stands for Fleet Ballistic Missile (Fleet Ballistic Missile Submarine). The combined "BM" was translated into a Navy nickname, Boomer. It's similar to a BMW automobile being called a Beemer.

The fact that ballistic missiles go "boom" when they explode also played a role in the slang term. Otherwise, they could have been called "beemers" also.