Whenever something odd suddenly starts happening inside Premiere, the most likely culprit is a corrupted Preferences file, which holds all the basic settings used by Premiere including user interface and other aspects like capture settings.
To reset the Preferences to factory defaults, hold down CTRL+SHIFT and start Premiere. You can release the keys when the splash screen appears. Then set up the interface as you wish again and rest scratch disks correctly before resuming normal operations.