This makes the nodes sorted alphabetically both in the add menu in the node editor, and in the node link menu in the material properties. Previously in the material properties the nodes were sorted in random order.

This makes things consistent with constraints and modifier menus for example. It does break muscle memory for those used to the current order, but in my opinion alphabetical order is better. I often find myself scanning the menu up and down unable to find the right node immediately.

For some earlier discussion see here:
https://lists.blender.org/pipermail/bf-committers/2014-June/043817.html

This does not affect custom nodes, for example from external renderers. We can't automatically sort them because addons can define arbitrary menus, with separators etc. So that choice is still left to the addons.