. So my question is how and why did this hive survive? They are a brownish yellow colour and not mean but more aggressive than my other hives have ever been.
What makes you think they did survive? An empty hive with a whole bunch of comb is an awfully inviting target for an early spring swarm.
CCD pretty much exactly tracks the movement of varroa across the globe. Its varroa, and the viruses it spreads. You have a nice strong hive building up, gets a relatively high mite load, and swarms. The swarm takes more than half the bees, a good chunk of the reserves, and very few of the mites, so you end up with a small, weak colony, a virgin queen who isn't laying yet (and maybe has viruses), no queen pheromone, no eggs, no brood, and a ton of mites. Diseased bees fly away and die, this increases the mite load on remaining bees, and they can't protect the honey they have. Only takes a couple weeks to go from a big strong bustling hive to a bunch of empty boxes with no honey, brood, or bees.
Pesticides are a red herring. They're clearly causing problems, but pretty much every study done on CCD points to mites, and secondary viral infection.