Sounds like the pond is very rich in nutrients, and you had a blue-green algae bloom.

It was probably not caused by the direct piping, though I suppose some nutrients could have been absorbed on the way to the pond if it were not direct. The real question is how the nutrients got there. Was the field fertilized? Do you fertilize the pond? Too much food?

Send some water off to be tested and see what you are fighting if you don't see an obvious problem.