What makes Lady Dedlock's past such a dreaded secret? Why do they go to such lengths to hide it and why is it threatening to have it revealed by Tulkinghorn?

have it revealed by Tulkinghorn?

In the era this story was written, women who had affairs with someone to whom they were not married were considered the lowest kind of human. The fact that Lady Dedlock not only had an affair with Hawdon, but bore his child as well, would threaten her very elevated position in society and would force hr to live in shame, perhaps even to lose her fortune.