pkgsrc does things a bit different to most other package managers. When a package is rebuilt using our bulk build mechanism, every other package that depends on it is also rebuilt. So for example, if we upgrade OpenSSL from 1.0.2m to 1.0.2n then every package that uses OpenSSL (and every package that depends on those packages, and so on..) is rebuilt.
We do this to ensure consistency and avoid any nasty surprises. Just because an upstream thinks the ABI is identical it doesn't mean that is true, and you cannot rely on this behaviour. By rebuilding everything that could be affected we have confidence that at least the changes have gone through a build test, and we're not just hoping for the best.
This has served us well in the past and shown up issues that other package managers have been unaware of.