From what I've read, it's actually both humidity and drainage.
According to Fine Gardening: "Humidity is thyme’s chief enemy.... Unless you have a site with excellent air circulation, avoid thymes with woolly or hairy foliage, since they are most susceptible to humidity-induced rot."
They tend to "melt out" during hot and humid summers. In fact, I've lost most of the thyme in the summer when they just melted away, even during droughty summers. I don't think I've ever lost one in the winter. I follow a local blogger who writes for GardenRant.com and lawnreform.org, and she also has lost most of her thyme in the summer due to them melting out.
Thankfully, culinary thyme has done really well for me because it doesn't have the kind of leaves that make it difficult to transpire water.
