It's caused by high DOC (dissolved organic carbon or less precise dissolved organic compounds). A fancy name for a certain size of organic matter. For example when a leaf decays it breaks into smaller bit and those bits decay and break into smaller bits. When bits get to be smaller than 1/2 to 1/4 of a micron (bacteria are about 1-5 microns) we call the bits dissolved organic carbon. The reason for the fancy name is because when these bits get this size they start acting differently and one of these differences is it starts acting like soap (surfactant).

Ponds often have a pretty high DOC level because there's a lot of decay. Generally over years DOC issues can increase as decay accumulates. But there are causes that can increase DOC levels faster. Green water algae is in the 5 micron range so they have a head start on decaying down into the DOC range, but it still takes awhile. Spawning can also add small organic matter which can increase DOC levels fast.

In ponds DOC levels causing foam can last for a day, a week, a month, it depends. But once it happens it's likely to happen again maybe once a year unless something changes. These tiny bits continue to decay down into elements so the foam can and often does disappear.

Most ponds with waterfalls have some foam on the surface. In most cases it doesn't last very long. I personally don't like the look of any foam, but many pond owners at least say they don't mind the look of foam on their pond.

Some of these tiny bits are in the foam so removing the foam can reduce the outbreak. A filter called a Foam Fractionator can be used to remove the foam. However, for many ponds, it's better to start getting serious about better water change management.

When I build a pond I always add a catch basin under any falls. Very cheap and easy to build. It can be added to an exiting pond. Here basically how it works.
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The foam stays trapped inside the basin.

Here's how one looks when installed. You can see some foam in the basin.
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There are other pluses to a catch basin.

Pretty much whenever I visit a Koi Pond the owner will say "Just a minute" and will then turn off pumps and bubblers and we wait until the pond surface calms down so the owner can better show off their beautiful koi. Koi are best viewed when the pond surface is like glass, without the distortion of ripples (and certainly no foam). The catch basin allows me to have a waterfalls and a calm surface free and always completely free of foam. Here's the same pond with the catch basin, notice the calm surface. You can also see that in the first photo that the pond surface right next to the falls is clam.
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Another plus is increase O2 (gas exchange) in the pond. Many people think waterfalls increase O2 in the pond, and they do but not very much. Water from the falls doesn't penetrate very deep. Most of the water hits the surface and spreads out. Well the water near the surface of the pond is really already maxed O2 so water from the falls really doesn't change much. The catch basin diverts all waterfall volume to the bottom of the pond and acts like a free TRS. So max O2 water from the falls goes to the lowest O2 water in the pond at the bottom and pushes low O2 from the bottom toward the surface where gas exchange happens. So the overall O2 level in the pond is greatly increased and it doesn't cost you one cent for extra electric.

The outflow from the bottom of the basin will sweep the pond bottom clean for a few feet out from the basin. That can make cleaning a little easier.

Lack of waves from the falls allow water lilies and floaters like water hyacinth and water lettuce to not be pushed around by the falls.

A catch basin can be turned into a pretty good Moving Bed bio filter in the future if needed by just dumping media into the basin.

I consider a catch basin to be essential in my ponds.

Thanks for the info Waterbug! I may have to incorporate something like that in the pond addition. I may have gotten a lot of DOC when I killed off a bunch of string algae type stuff that was growing all over the side of the liner a while back....my entire liner was green and hairy about a month ago. It looked pretty cool but it was going to take over everything so I used algaefix to get rid of it. It probably was not string algae, but it was very thick and about 6 to 8" long...the fish loved it.

Better than algae fix - stop feeding your fish and let them clean up the algae!

Lisa---I cut way back on their food. We like watching them eat. Maybe I will cut back a lot more though. :)