The tail is dirty because its the APU compartment. It gets warm when the APU is used and any dirty water is going to evaporate and leave any residue behind far easier than the main fuselage. There's a sudden transition in dirt because of the firewall doesn't transmit the heat much further forward.
Airplanes don't get cleaned much these days, for various reasons, cost, environmental considerations, fleet size, etc. hence there are a few dirty looking aircraft around. Google images of a B747 and you will soon see evidence of a hydraulic leak from a rudder.
All I see here is just dirt. Skydrol or grease are most likely in the mix too, there are lots of grease points up there and it will get washed down. The transfer/paint scheme is deteriorating but that is not unusual especially at leading/trailing edges and areas where airflow vortex and turbulence occurs. This is not an indication of poor maintenance, a defect or the back of the plane falling off, but purely an aircraft that has obviously been used a lot.
Last edited by Filler Dent; 9th Nov 2015 at 13:20.
Reason: bad grammar