Fearfull Lab-Rottie Cross!

I have a very large black lab, rottie & Burmese cross. He is 2.5 years old and lately has been showing signs of fearful aggression. He is very well trained, can respond to any command, from sitting, lying down, healing, staying, no, yes, Etc. and is a wonderful dog. On two occasions when being tied up in a very busy area, people have gone up to pet him and he has responded by aggressively barking and snapping. It doesn't happen all the time, some times it will be weeks before he does it. He has also growled and nipped at a man, off leash. It always happens to strangers and if I am not directly beside him. As if he is protecting me from strangers. I want to know how I can fix this problem, so he can be approached by strangers.

Well something you mentioned really hit home, you said he does this in a busy area. It sounds to me like a case of overstimulation while being left alone in a strange place with strangers coming up to pet him. I'd be snarly too. If you can avoid tying him up by himself, I would, it sounds like you've got a ton of really sketchy variables that are coming together to make one rotten equation with a very scary end result. I would recommend keeping him with you on leash at all times, and if you're in a spot where you'd be forced to tie him up without you right by, then leave him at home. If he's unreliable off leash, whether his intentions are good is immaterial, he can't be off leash until you're 100% confident that years will go by without an incident. There are trainers that frequent these boards that may give different advice and much better than I, I just know how my dogs react :)

How old is he and when did this behaviour start. Has he always been fearful of strangers. Is he fearful of anything else.

First of all I would like to ask why in the heck is he tied up alone in a busy area. And also why do you allow strangers to pet your dog. Dogs, of any breed feel more protective when tied up, whether it be on leash or on a rope. That is how most attacks happen. At this age. It is just the end of adolecence. Get him back into formal training, and quick. Do not tie him up, and do not allow people to pet him without you right next to him....
With all that said in done.... Good luck, does not sound fearful, sounds like I am too big for my britches....