Why does this happen? It's driving me nuts.

[CloudWatcher]

3. Sounds like a corrupt preferences file

If the other suggestions don't work, here's a more brutal fix:

1) In Finder, use to the "Go" menu and then "Go to Folder" commands to go to "~/Library"
2) You should see a lot of files and folders, one of them will be the "Preferences" folder, open it up.
3) You should see "com.apple.dock.plist".
4) Drag it to the trash and logout and login again.

Warnings:

1) this is deleting your dock preferences, so the dock will reset to the macOS default. You'll lose
any customizations you've made to it.

2) there are lots of files in ~/Library that are not safe to delete. That's why it is "hidden" by default and
doesn't show up unless you use the Finder's "Go to" menu to go into it.

Hope this helps