Technical answer, there's no planned Reaver/Super Mutant encounter. While the engine can handle a random encounter where two sides are fighting, what you are finding is really two seperate random encounters at the same time. In the random encounter set up you can specify the alignment of the people you are encountering, good, bad or critter. Critters tend to fight everyone, Good alignment will set people friendly to you but Reavers and Mutants are both set to Bad so they are hostile to you. Since it's not a planned meeting the engine is probably treating them as being on the same team and friendly to each other. Though this is all conjecture since I've never had such an encounter.