Enough water comes in from opening the door in the rain.
Even though your carpet may feel dry it is still damp down in the pad. The floor mat does not allow the carpet and under pad to dry. Even a little moisture will show up on the mat from condensation. Water was on the bottom of my mat in the same spot. Even though the carpet felt dry, when the mat was in it would have the damp spot on bottom after driving.
I had to drive it for a time with the mat out for it to dry completely. So whenever I open the door in the rain or whenever water drips in I remove the mat for a day or two to be sure it dries.
It ain't easy being cheesy:
Sometimes I make decisions like famous people.
George Armstrong Custer "lets go over that hill, I think those are friendly indians!"
Titanic's Captain Smith "What icebergs? Full speed ahead!"