When exposed to a loud sound, the human ear will react by contracting slightly, temporarily reducing the perceived volume of sounds that follow. This reflex maneuver, sometimes called "blinking" (by analogy to the eye), is meant to protect the delicate structures of the ear from potentially damaging sonic power. However, it also means that relatively loud sounds in an audio signal, such as a loud trumpet's note, will tend to overpower other sounds that occur just before and just after it, a phenomenon known as temporal masking.
It may seem strange that a masking sound can drown out another sound that arrived at the ear before it, but one must realize that the ear is by no means a perfect sensory organ-- the eardrum responds to a collection of sounds and there is a window of time in which an incoming sound can be masked by others following it. The window for masking is wider for masking sounds which precede the masked sounds, but the window for "backwards" masking is also significant. However, this type of backward masking is not to be confused with the backward masking used to hide time-reversed secret messages in sound recordings.