Troubleshoot Dock behaviour in Mac Yosemite
Answer for my own question...

I mistakenly changed the default application for opening folders (!) away from Finder to an application of my own creation. I didn't know this was even possible! Needless to say this had some weird side effects, like the problem I originally described. But I figured it out – I mucked up my system with an erroneous QuickSilver command.

I'm always creating dated folders, so I created an Automator app that, given the current folder, would create a folder within using the date as the name.

When teamed with QuickSilver's "Current Selection" trigger, the app becomes useful. I can call my "create-dated-folder.app" from the current folder, using "Open with..." in Quicksilver's second pane, and my app in the third. Works great.

My mistake was, instead of "Open with", I once chose "Always Open With", which changed system settings I didn't know existed – it changed the default application for the "public.folder" UTI from Finder to my Automator app. Then the weirdness began, like if I chose "Open Examples Folder" from Automator's help menu, my app would be invoked. I really had no idea what was going on!

Brilliant tip from to Tetsujin who suggested RCDefaultApp would be worth investigating.