Maybe depression is too strong a word, but massive "anti-climax" plus exhaustion fits.
People work on a major thesis like MSc or PhD and by the end of it they're absolutely lost in a micro-world. The day after graduation more than a few students turn up for a "what the fuck am I going to do with my life now?" crisis tutorial.
I've seen authors and games developers get the same, inclusing myself. After 6 months of "crunch time" they just run off a cliff like Wile E. Coyote and keep running in mid-air before plunging. Their manager knows they're finished but their own nervous system hasn't got the memo yet.
For cyclic stuff like software releases I suppose that can also be mixed with the dread of what is to come on the next round. In bigger firms I've seen clever managers move people around between projects in overlapping waterfall, so they don't get too emotionally invested in the "the big day" for "their baby".
Some people are looking for confirmation from others to validate their own self worth. Some might work super hard to "prove themselves" (via others). If they recognition they desperately need is not forthcoming, they are quite vulnerable.
Some are desperately looking for a purpose, and hope that $NextBigThing will be it. When they realize it is not it, they are quite vulnerable.
None of these pathologies are particularly unique to developers, I think. Can happen to people in many kinds of professions.
That said, I noticed this "down" effect earlier in life after completing something challenging, like passing a hard exam, etc. But not with software.
If anything, we should stop sometime to reflect on what we achieved after longer period of time, it's too easy to be on a treadmill and never feel that anything significant is finished and accomplished.
To your point, I've read that mountain climbers and long distance hikers experience "post trail depression". I guess a goal oriented mind is adrift without something to work towards.