Old jets, acid rain, poor maintenance -

http://www.bbc.co.uk/news/world-us-canada-12946808




Jets parked outside are subject to acid dew and rain and they require constant corrosion control to keep them structurally sound.

Note that these failures are almost always on the top of the airplane where the acid dew falls.

Airlines try to cut corners to keep fares low and this is the result.