Have you ever had a phobia develop or strengthen for no reason?

This morning I was in the bathtub, thinking about NaNoWriMo and idly playing with the water. (I was flicking the surface to play songs. It’s fun!) Suddenly I realized that I was feeling nervous because of the water.

I’ve always had a slight fear of water, but only when I can’t see or feel the bottom. If I can see the bottom, depth has never bothered me. If I can feel the bottom, murkiness has never bothered me. My bath was certainly not that dirty and the tub is definitely not deep enough that I couldn’t touch the bottom, so I should not have been afraid. Through a little experimentation, I discovered that I was fine if I lay back and relaxed; I was only nervous if I sat up and looked down into the water. But I still don’t get it. Nothing has happened to me that could explain this increase in my phobia.

Has anything like this ever happened to you?