Clicking app from dock opens finder window. Won't boot in safe mode.

I tried to launch an application from the dock, but instead of launching the app or switching to the app, a Finder window opened, open to the Applications folder. This happens with anything I click on the dock.

I restarted, but same problem.

I then shut down completely and re-booted holding the shift key, but it seems to have booted in normal mode. (I opened "About this Mac," system report, software, and the boot mode said "Normal.") I tried this twice.

Other strange behavior is in Safari, clicking on any link opens the link in a new tab. I also can't seem to type in this window when other windows (like About this Mac) are open. It's as if I can't click on a window in another application to switch. I can only seem to switch applications by option-tabbing.

Any suggestions? Thanks very much in advance.

iMac 27" mid-2011

OS Sierra 10.12.4

3.1 GHz Intel Core i5

20 GB memory

AMD Radeon HD 6970M 1024 MB

Booting from an SSD. I had replaced the optical drive with an SSD last year.