Furby thinks his shaking.
Furby thinks it shake in the vertical position and therefore always turns into the evil.
How to get your Furby to change personality?
Default Personality. All Furbies start off like this. You can get this personality back by turning your Furby upside down, holding down the tongue, and holding the tail pulled for 2 seconds.
If your furby thinks it shaking while in the vertical position and switches its personality to evil/Feisty/ Viking mode. The furby tilt sensor is broken or too sensitive. For instance, if the furby dances, he may stop, since he thinks he's being shaken. I advise to take apart the furby to clean/replace his tilt sensor
This site might help even though it's for older Furby's:https://furbytech.tumblr.com/post/167541...