Today I was reminded about a very important lesson that I learnt when I was a child.
I was with my Dad out looking at horses for sale in a friend's stable-yard and there was a lovely Border Collie tied up to a kennel. Being an inquisitive 5 YO, I wandered over to give this dog a pat and was immediately bitten on the hand. I was lucky that day that I was only frightened and not hurt. A friend's little boy, who had the same thing happen was not so lucky. He ended up in hospital to have his face stitched back together.
The moral of the story is don't pat strange dogs that are tied up.
A dog that is tied up immediately feels at a disadvantage. In an insecure dog this can cause aggression. He may feel threatened when approached by a stranger, especially one who ignores his body language messages. He may be telling you that he is not comfortable and to stay away and if you cannot read the signs you may be in for trouble.
On another note, it is actually rude to pat another person's dog without asking permission first. Be respectful.