The "ancients" (Roman era writers and workers, post Roman UK Peak District lead workers, the Chinese, etc) understood the hazards of acute lead poisoning first hand and often referenced mad hatters, crazy painters, and the symptoms of extreme exposure in literature.

What they seemingly failed to grasp until more recent times was the slow creep of incremental mild exposure:

  However, they were so fond of its diverse uses that they minimized the hazards it posed. Romans of yesteryear, like Americans of today, equated limited exposure to lead with limited risk. What they did not realize was that their everyday low-level exposure to the metal rendered them vulnerable to chronic lead poisoning, even while it spared them the full horrors of acute lead poisoning.

> what would these forboding circumstances have been attributed to at the time?

I’ll bet witchcraft got brought up, and praying to the wrong god or going to the wrong church.