I know exactly what you are talking about, weed, we live out in the woods. It must be a guy thing.
I'm guessing it was most likely a wild dog or fox, something with a good sniffer looking for food. That's why whatever it was kept digging passed the fertilizer spike, they were searching for something more tasty, like a ground mole, vole, or other ground creature. And, that's why the urine worked to deter them.
