From what I can see It's because that section of wick is both very much shorter than the other side of it AND it's right close up against the cap of the RDA. With the central side being so much longer you're going to get more
juice wanting to go up into the coil from that longer side. Also, when the wick sits that close to the edge of the RDA it soaks up all of the condensation vapor that is on the side walls of the top cap which tends to be a darker color, the longer the device sits without using it the more chance those wicks have to soak up all of the stuff that clings to the RDA cap and runs down the walls.
Basically the juice is just sitting there in that portion of your wick getting heated over and over, it doesn't really have anywhere to go when the cap is on.