Sometimes due to various reasons, some snow crystals do not bond well with older snow, forming a weak layer in the snow pack. As more snow falls the weight on top of the snow pack becomes enormous. All the snow is waiting for is something to trigger it. A trigger can be man-made or natural. The best way to explain a trigger is to visualize an old rubber band.
Snow is like a rubber band.
It likes to stretch back and forth. If the old rubber band is pulled too fast and quickly, it will snap. As with snow, if it is not allowed to slowly move and bend, it too will break at the weak layer. By overloading it at the precise moment it is trying to move and bend, it breaks. This overloading is known as the trigger.