The Reason Why
In the previous year, 2018, for the first time in the backyard, monarchs laid eggs on the milkweed. Eventually, four caterpillars developed and went walk-about to seek places to make a cocoon. We lost sight of two, but two successfully made a cocoon on the fence. The first of the two was attacked on the day after and drained; the second wasn't viable. In brief, Sue and I had no luck whatsoever in 2018.
We decided that if monarchs should again lay eggs on milkweed here, we'd do better for the butterflies.
The warning: July 21, 2019, was the warning to get ready. I had taken a casual photo of a female monarch, and Sue pointed out (something one can barely see) a tiny, tiny caterpillar on a leaf (seen beneath the monarch's wings).