Atmospheric pressure is constantly pushing down on the water within your tank. Because the tank holds water, it has nowhere to go.
When you add a siphon hose, it is commonly thought of the water being "sucked" out of the tank, when in actuality, the water is being PUSHED out.
The atmospheric pressure pushing down on the water plus the siphon hose gives the water a path to travel (out of the hose).
But the atmospheric pressure isn't enough to push water in an upward direction against gravity, so, it is important that your siphon hose empty out to a container completely below tank level (the top of the bucket should be BELOW the bottom of the tank).