How to disable automatic copy when selecting text from the keyboard?
Bah, it turns out that parcellite was the culprit. Uninstalled that sucker, now everything is back to normal.
Indeed it originates in Parcellite, but you don't necessarily have to uninstall it. You can go to: Preferences (default shortcut is Control+Alt+P) -> Behavior ; Un-check the "Use Primary (Selection)" checkbox.