the mirror inside the camera, which allows you to see through the lens, must raise quickly in order for the camera to make an exposure. the vibration from the mirror hitting the top of the chamber is significant enough to affect exposures less than 1/30s. after about 2s, the vibration becomes increasingly insignificant because it is very short relative to the total exposure time.
when using MLU, the camera should be on a tripod and the shutter should be released using the self-timer or a remote release. releasing the shutter using the shutter button would defeat the purpose of MLU because the act of pressing the shutter introduces vibration.