I've been thinking about this, and I figure Salty came from that (wow wtf is Guile doing?) feeling you get when you're cooking...and you go to put a little bit of salt into the pot of food, and the cap comes off and the entire contents of the salt container all go into the pot at once.
You've just ruined your dinner, and you feel extremely stupid.
Literally salt!
I remember the white kids didn't get it too.
They'd say it like, "You're so salty, you forgot to wipe the salt off."
BTW
Salty to this day still means that you're feeling stupid inside.
If you lose a game, you feel stupid, outplayed or whatever. So Salty is the right word...just the people using it don't know exactly why they're using it, or what it really means, and when you try to enlighten them, you get stuff like...
"What you need to understand is..."
No one likes being told they're wrong. No one likes being enlightened to a better way of thinking, or truth.
It's a very harsh reality, but telling a commentator what salty really means is extremely useless, unless he's an open minded nice fellow.