I can think of two possible reasons for what you are experiencing. One, is there a stand pipe from the bottom of the pond that once the level of the pond reaches to the level of the top of the stand pipe it over flows and the suction siphons the pond? Like a toilet.

I used the term stand pipe but it would not have to be an actual pipe only something that acted like a pipe, possible some wrinkle or over lap in the pond lining.

The other is similar but can be proved only by logic. A couple of times our pond has emptied. What was happening was the electricity was going off, the pond was flowing to the pump box causing it to overflow. When the electricity came back on there was not enough water in the pond to maintain the flow over the weirs. The only way that I figure this out was the clocks in the house had to be reprogrammed after the pond problem.

You might start trouble shooting by finding where the water went after it left the pond, (wet spots in the yard) and the level to which the pond drops.

This is interesting. Any chance of pictures? How the pond is set-up with pumps and filters.

rcharles,
The problem is the water in the ground around the pond is higher then the water level in the pond. This can happen one of two ways. The first is if you are pumping water to a water feature such as a waterfall or fountain or external filter and the water is leaking out there. If it is flowing back to the hole in the ground where the pond is. The water level in the pond is going down because of the leak but at the same time the water in the hole is filling up. When the water in the ground gets to a level that is slightly higher then the water in the pond the liner in the pond starts to float like a boat. As more water is pumped out of the pond it is replaced by water on the outside of the liner until the pond is empty. Normally the water in the ground around the pond soaks into the ground and can't lift the liner unless the ground because rain can't hold anymore water. The way is for rain to raise the water level in the ground higher then the water level in the pond. When this happens the water level in the pond rises. It could rise high enough where it could flow out over the edge of the liner until the pond is empty. This situation is a lot more rare then the first and requires some unusual circumstances. The pond has to be built on sloping ground. The liner on the lower parts of the slope the has to have a way to let the pond water out and the ground water not back into the pond. So usually the liner follows the slope down very close to the surface with usually a fold as to act like a drain tube while the water in the ground can be higher then the water in the pond. This is rare but can take place.
Mike

Mike, I've seen what you describe happen, if fact my tubing came loose and caused it to happen. But he said he had weighted things in the pond. ???

Bubba,
All that means is the water on the outside of the pond has to be higher as compared to the water level in the pond. This would not be the case if it wasn't for the liner. The liner can and will act like the hull on a ship and then water can lift objects that are heavier then the water itself because the psi of the object is not as great as the psi of the water around it and above it.
Mike

Wonderful to get this response.
It is difficult to describe and if I can post pictures I will. I have never posted pictures on this website. Has anyone done this? Is there a posting, giving the particulars of posting pictures.
I may not be explaining it correctly. I do feel that it is definitely something under the liner pushing the water out as it rises. It is gradual, but to the point that it empties.
We did take out most of the water plants(quite heavy) this past fall, after the first time that it emptied the pond.
We have had fish(Koi) in the pond since it was built(15+years), but in the last two years mink have gotten in and eaten everything. I cover it with a black wire mesh(not heavy enough gauge) and they got in and ate everything again this past fall.
I mention this as I was questioning whether I want to keep stocking this pond with Koi or gold fish, only to feed the mink.
So, we emptied the pond of plant life as we did not want to fill it with water.
Now it sits with just the pots and slate at one end.
Interesting though that the pond filled up with rain water and there is no run off helping it to fill. It was overflowing slowly into our bog area (lined). Could not see it flowing.
This could become very long winded and I think if I can post pictures it would help immensley. I will look into it.
Thanks for all the imput.

Here's a link for help posting photos from over in the gallery. Hope it helps.

Here is a link that might be useful: How to post pix

Mike il explained it perfectly. It is just the physics of water. Here is an experiment you can try at home to demonstrate the concept.

Get a bowl. Get a ziplock bag that will fit inside the bown and ideally be large enough to cover the rim. Place the bag inside the bowl and fill it about 1/3 with water. Next fill the bowl (not the bag) slowly with water. As the water level of the bowl surpasses the amount that is in the ziplock bag you will see the bag start to lift and evenutally it will lift enough and start to spill water out on to your counter. The ziplock bag is your pond and liner when it is not full. The bowl is the ground and you are observing the effects of rising ground water on your pond.

Now try this experiment with the ziplock bag filled up to the level of the bowl. When you add water to the bowl it will spill on to your counter but the ziplock bag (your liner pond) will not lift and will stay intact. So a liner pond that is filled with water to ground level will not lift and spill water unless under the most unusual of circumstances.

Lisa