The reason this happens in real instrument sounds is basically simple physics. Say we hit a string, or a membrane, the first excursion of the string or membrane is the loudest. After that, it springs back due to elasticity, and it's losing energy all the time - pushing air around & internal friction. It's the same principle that a pendulum, once released, will never return to it's original position.

Wind instruments and vocals are under the control of the player/singer, but there will still be some asymmetrical component - especially with sibilance and plosives.