The developers have a release cycle that includes periodic hard forks. AFAIK there is nothing algorithmic to entice or force a fork like the Ethereum difficulty bomb does.
The scheduled hard fork is announced weeks in advance, and high-profile clients like exchanges are "strongly encouraged" to upgrade. If you run your own XMR full node on an old version past the fork date, you get a bright red warning message telling you to get the new version.
That sounds extremely centralized.
Anyone can be a Monero contributor and can work on the hard fork.