Phobias are usually down to what we call an intial sensitising event. A one off leaning experience coupled with strong emotion.

Often this will involve a fear of death either to self or a loved one and usually occurs at a young age (often between 3 and 8) when the child has not had time to build resources that could deal with the situation objectively.

I repeat from the hypnosis page (in case you haven't been paying attention) the example of a woman who presented with a fear of water that was seriously curtailing her social life. In trance she accessed an image of her mothers face contorted in fear and screaming at her and her sister to get out of the water (a shallow river near their home) or they would die.

She was only about 5 at the time so the memory had been covered over (probably because it involved humiliation, invoking a sense of shame and being caught doing something wrong) but once brought out into the light the fearful emotions were released and it became just another childhood memory.

Quite possibly as the woman grew older when she looked in the mirror she was reminded of her mother and the phobic reation was triggered that much faster. Avoidence of water scenarios over the years had the effect of reinforcing and strengthening the phobia to the degree that the body would now exhibit stress symptoms (sweaty hands, shallow breathing) at even the thought of going near water.

Substitute spiders, wasps, slides, birds, feathers, dogs,horses or just about anything that exists in our daily world for water and play about with the scenario.
But intense emotion and fear of death or punishment (just as scary to a small child) will always be involved - remember the reason phobias are hard to understand is because they don't make sense to the adult mind.

The good news is once we neutralise them they are gone for good