SEM Lab, Inc. has seen the same type of resistor failures repeatedly since ~ 1997 when we were founded. The trigger appears to be the local environment where the resistors are deployed, for example ...
A computer module in a tractor.
Fireworks display controller. Boards coated with ~ 0.300 inches of Dow Corning "tough gel".
A display at an outdoor drive-up kiosk.
Many other previous analysis efforts where the source of sulfur is unexplained.
The problem, sulfur pollution and corrosion of silver, is illustrated in Fig. A and explained thoroughly in references  & . The resistors increase in value and eventually fail open.http://www.semlab.com/resistorcorrosion.html