If I were to speculate with item 1, in a clean room you require...

If I were to speculate with item 1, in a clean room you require continuous positive pressure to keep particles from seeping in through any air leaks in the entire room/area. Allowing fans to turn off for a moment allows particles to leak in, and also allows particles in the air to settle onto surfaces, which makes it more difficult for them to be picked up in the circulating air for filtration.