I've Googled it; it seems that post-race depression sometimes occurs after a big event - you've trained so long for so hard, juggling work and family commitments with training for race day - then you reach your goal - and suddenly you don't know what to do with your time - mission completed, and your life seems empty.
My personal totally unscientific theory goes more like this:
You have a tube of happy hormones in your brain. You go run or cycle a stunning piece of singletrack, or smell good coffee, and the happy-hormone-tube is being squeezed a little bit and you go on a little high. You go do a nightride on gnarly singletrack with gauteng's coolest crowd, and you drink hot chocolate with friends afterwards, and the tube is squeezed a little more and you glow with endorphins (or serotonin, or dopamine, or whatever causes this high) for days.
The total stock of happy-hormones in your brain is now less, but you operate off the released happiness while you sleep or work or drink good coffee or clean your bike, and over time it replenishes itself, so there's always stock when something squeezes the tube next time.
All working like nature intended.
The problem come in if you squeeze the endorphin-tube too hard without giving it time to replenish.
e.g. you go do Swazi Xtreme. You sleep very little, but you spend time with some of the best navigators you'll ever meet - you cycle in the dark, you trot on a dirt-road while talking to buddies in other teams as they pass you, you catch up with other team's seconds at transitions, you paddle on pretty rivers, hike in a kloof, carry your bike over gnarly downhills, find controls in unexpected and pretty places - and you use up a lot of happy-hormones. Then, while you're still on the high, you have 3 days to sort out the bike-issues and light-issues that broke on Swazi, get your adventure gear clean, get in the car - and start squeezing the tube again as you cycle down into the Baviaanskloof, as you catch up with friends along the way, as you plan where the controls for an adventure race might be, as you manage to get up the mother of all climbs without putting a foot down, or fight the sleepmonster, or almost get blown off your bike by a very temperamental sea-wind, or reach a checkpoint and can sleep for 4 minutes, or get to cycle some awesome dual-track at night by the dim light of your headlamp, or hear the sea, or cycle into Jeffereys bay 230 kms later - you're on a high, and the happy-hormone-stock in your brain totally depleted.
Like an empty toothpaste tube, it won't matter how hard you squeeze the tube. Empty is empty.
You'll just have to wait it out.
Eat some chocolate, email some friends, go buy pink flowers at the nursery for the friend's newborn-girl, drink some good coffee, go watch Evita at montecasino - even dine at Melrose Arch with some Ausie-friends that you haven't seen in years - but don't expect it to release feel-good feelings.
Lie awake for hours at night, search for company on the hub at 3 in the morning, sleep late every morning - till 12 on weekends, and 10 on weekdays (luckily your work allow that) - get a cold, play mindless computer games. Go find a paddle shop so that you can replace the paddle that went missing at SwaziX. Have a haircut, eat some chocolates. Bleh.
I went for a ride to Albert's farm on Sunday afternoon - first time i've done anything since Transbaviaans - all on my own - and the singletrack was pretty and dry, a little dusty - but lots of happy people at the botanical gardens, the downhill into delta park fun as ever - and i think the happy-hormones finally replenished. Managed a run as well on monday afternoon ... the sky is blue, the Spruit smells of jasmine, and I'm doing a sprint with Dawn & Zoo Cookie on Sunday - life is good again!