Why do relays burn out? Is a burned relay indicative of any electrical problems in the car or just an isolated problem?

I was driving down a hill, happy as a clam, when my car died. Making a long story short, I found a burned relay (part # 898160) that is green in color and slightly to the left of the steering wheel. Replacing it solved the problem. The #30 metal lead, in the upper left corner as you're looking at the leads, was burned, with the wire from that lead going into the cylinder. I resoldered it, but don't know what went wrong.

On the side, I had spare relays, which were black, and had different part numbers (#8988158 and #3523610). I thought that these junkyard relays had come from identical Volvo junkyard cars that I retrieved from the same area. I didn't install the black relays because the part numbers were different and the little numbers next to each metal contact were different. What do the black relays do? Thanks in advance.