Does it look like that big dog just wandered over to 'say hi' and Tinydog snapped at it totally unprovoked? Somebody obviously doesn't read dog body language very well. Let me show you what you missed.
A dog with good manners would have picked up on Tinydog's desire to be left alone already.
At this point the other dog is either deliberately ignoring Tinydog's attempts to communicate, or just has no idea how to read a dog that small.
The oncoming dog is moving in a straight line with ears and tail standing up, this is not a friendly hello, it's an "I'm gonna check you out and there's nothing you can do about it."
So neither Tinydog, myself, or the other dog are surprised when this happens:
My chihuahua is not Cujo. He is not vicious, and he is not attacking unprovoked. He actually has pretty good manners, and he only snaps at other dogs when they ignore every single lesser attempt to communicate that he wants some space.