RRoD was caused by an overheating issue related to how the internals of the system used to be designed. Basically, there's a clamp that holds a couple of the parts together, but it's really cheap, and it bends really easily when the system heats up. If it bends too far, wham, RRoD.
It's worth noting that the PS3 had the exact same problem, except it was solder melting, and Sony didn't offer free repairs for out-of-warranty consoles. Look up "Yellow Light of Death" sometime.