Snatch is that way....
he was a happy puppy, we got him at 10 weeks old.. he was not scared of anything...
we took him EVERYWHERE we could: famer's market, best buy, homedepot.. spending saturday mornings at petsmart just for him to see everyone...
and then around 9month he started... everything looked scary to him... a car with an open hood, someone dropping a box in the stairs....
all of a sudden.. and nothing bad ever happened to him
Snatch is 4 now. He's a good doggy

he learned a lot and is ok now to see a car with an open hood

I just know that my doggy can run off because of a loud noise....
Same if a stranger enter the house.. he will run low until his safe place (his crate) and stay there with his head popping out...
he's ok when the person sits.. then here comnes the giggles :D
But just imagine the maintenance guy... he's scared of dogs.. and my dogs is scraed of him... it's like a circus when he comes in... LOL
I don't force him.. I just ignore the behavior.
I though we socialized him well... i don't know what happened.. he's just a scarty cat....