The roots of this go all the way back to 1942, with the release of the movie Cat People. Cat People was a horror film produced by a journalist/novelist named Val Lewton. It told the story of a woman named Irena who believes she is descended from a race of people who turn into cats when aroused.
During production of Cat People, Val Lewton coined the phrase “Lewton Bus.” Lewton is also often credited with popularizing its usage in horror films. The terminology comes from a scene where Irena is following a woman named Alice. The audience is meant to think Irena will turn into a panther at any moment and attack, but when the camera focuses on Alice and the tension rises, the sound of a bus (sounding like a panther) pulls up.
The term is today (though clearly rarely) used to refer to any scene where building tension is dissipated by something startling the audience.