As an ENFP has a wild child switch, ISTPs have just as much an emotional switch I think. We become emotionally unstable because we dont want to think about how we are feeling and try to keep it bottled up. Numbness is better then being overwhelmed by the strong feelings you can have for someone. Believe me as an ISTP, you can truly have strong feelings for friends and family, even though you will appear cold to others around you and even yourself for trying not to feel those emotions. When they do boil over, since an ISTP is so attached to the physical environment around him or her, it is natural that this overwhelming anger/sadness manifests itself in the ISTPs violent behaviour through some destructive or unproductive outburst of energy.

I dunno Ive had a lot of coffee this morning, just a thought from an actual ISTP . I hate crying.