There can be several of reasons for your "drinking problem".

You mention that your horses cannot graze any more. If you switch from grazing to hay that could cause them to drink more water because the moisture content in hay is much lower.

I also have found that grazing keeps them busy - and when they do not graze they just sometimes drink more out of boredom.

And I would not rule out deer or other visitors. Use a rake or broom at night and treat a fairly large area around the water - and check for tracks in the morning. You might be surprised who comes by.

Good luck!

GC