That rarely happens. In that case the incentive is actually to stay on, let your co-founders drive the value, and pocket the equity/$ for it. Easy money. Takes a rare breed to self-sacrifice for the good of the company.
It's more likely what annovikov said, the one leaving has serious doubts about the strategy but got out-voted.
What is the self-sacrifice in this case? You're trading "working to build a business to earn an equity-stake" for "free-riding on others' work to build said business to make your equity-stake more valuable."
Think of it as two separate companies: Startup A, which you co-founded, and which ended up dissolving, you seeing nothing for your efforts (other than whatever salary you managed to scrape out); and then, Startup B, which you bought a huge number of shares of for the amazing price of $0/share, and now get to smile as those shares appreciate, like any investor. It just so happens that Startup A and Startup B are the same company at different points in their life, but that shouldn't change your views toward the two situations.
It's more likely what annovikov said, the one leaving has serious doubts about the strategy but got out-voted.