and no I was no longer attached to the tree. I wear a harnass while on the stand, but once I am climbing I detach it.
Boo.
I've used a summit viper probably 130 times. I've NEVER had anything like this happen. Never. I actually don't see a way it COULD happen. Either the stand wasn't set properly....OR...the knuckle on the cable had to become detached (for it to slip). If your cable isn't broken....I'd bet on the former.