Lightning Strikes

I learned last night that Roni is deathly afraid of thunderstorms. Which is weird, because I'm fairly certain last summer we had some storms and he didn't seem the least bit phased. In any event, we were walking down the road yesterday evening and it was raining a little and then CRACK! a Lightning bolt lit up the sky. We were in the middle of the road, but that didn't stop Roni from trying to find something, anything, to hide under. Unfortunately for him, there was nothing in sight.
I got him home and inside as quickly as I could. He remained huddled by the door (we live in an upstairs apartment, so there were no windows here) where he was "safe" from the storm. I didn't even know my poor baby was scared until I went into the kitchen to get a snack. Roni got up and could barely walk he was shaking so hard! I got him to come over and cuddle with me on the couch, but he seemed more content lying at my feet.

The storm lasted for about an hour. By the time it was over there was a puddle of saliva left where he was laying because he'd been panting so much. I know my dog is not alone in his phobia, but why are they so afraid of thunderstorms?