It unnatural cases, yes. For example, if you put a bucket of snakes in front of a child, and a high powered rifle, the child would be more likely to play with the high powered rifle. There are things we fear naturally, like heights, and snakes, that are evolutionary traits we have gained so we don't die.
Irrational fears can be caused by many things, and I am certain that while there are fears caused by events in our lives, they are just to make us more likely to survive long enough to pass along our genes.
There are also fears that can be caused by conditioning. I can condition a person to flinch in fear every time they hear a certain word.
This can be accomplished in different ways, and I don't feel that I need to clarify the method.