They are arcing/carbon deposits left over from improper gapping/level contacting surfaces. Don't use dielectric grease on these contactor points. I've been one in the past who mentioned possibly using this here. Wrong. Only use this grease on fixed, connectors. Not on relay contactors or switch type connectors, that make and break contact.
These contactors must be filed/cleaned/and adjusted very straight and parrallel to one another. Very hard to do. Bend the copper arch tabs to much one way or the other, and the longevity will be iffy at best. Sometimes it may be just best to bite the bullet, and pay the $40.00 a pc. to buy all new switches.
Note, these switches must have all their inards aligned properly also (all three of them, the same way). The wiring thru the doors, is ALWAYS subject to adverse operation also, as, intermitant, wire fractures can cause much HAVOC in diagnosing.
IF, you find a real good, long term solution to this, I'd love to know.
Mechanical/electrical. Mechanically..........they fail, and are very difficult to get to come back into original spec. At least, thats been my experiences.
Good luck!