The series of systems moving through the Tennessee Valley, towards the Mid-Atlantic, have generally been rainy, but not particularly threatening. Not individually, at least.
The problem is that the stream of moisture took a path through the hills of eastern Kentucky and West Virginia. While steady rain anywhere will end up being problematic, it’s a much shorter trip to disaster in the terrain of the central Appalachians. Take a look at the video from WTHR showing the fast water flowing through the streets of Hurricane, West Virginia, west of Charleston