My computer running El Capitan (10.11.6) keeps resetting the apps used to open particular file extensions. For instance, it randomly sets PDFs to open with Acrobat Reader instead of Preview no matter how many times I've used the 'Open with' menu to set the app I want. Strangely, when I right-click and go expand the 'Open with' menu, Preview doesn't even show up in the list – I have to choose other and browse for it.
Possibly related: I've long used the Terminal to launch apps, using open -a 'name of app'. That's also stopped working for certain apps. If I for instance try to open -a 'ia writer classic' it tells me Unable to find application named 'ia writer classic'. If I then launch IA Writer from the Launchpad, close it, and try the command in the Terminal again, it works. At least for a while...
Any clues as to what might be going on would be very helpful!