Hi!! I've just registered on Macrumors just to comment in this thread. I had exactly the same issue in my MacBook Pro 15", base model with SSD updated to 512 gb.
When installing apps right after getting it out of the box, UI froze about 4 o 5 times. First, folders in the dock (Applications and Downloads) become unresponsive to normal click, they do nothing, but you can do right click on them, and the rest of active windows seem to work. Then, after some minutes, Finder freezes and you can't neither restart it, nor click on any window. You can do anything but hard reset. I found people reporting the same behavior in Apple Communities:
https://discussions.apple.com/message/31138961?start=20&tstart=0
FORTUNATELY, I solved it introducing these commands in Terminal, supposedly destined to clean Finder's cache:
rm -fR ~/Library/Saved\ Application\ State/com.apple.finder.savedState ~/Library/Caches/com.apple.finder; killall Finder
mv ~/Library/Preferences/com.apple.finder{,.backup}.plist; killall Finder
I introduced both, and then do some maintenance with Onyx. Never seen the issue again, in fact, my machine is running faster than it was as first. So... totally software related. Hope this helps you as well or Apple acknowledges the problem and gives a solution.