Why is my dog so scared now?

my dog is a minature schnauzer. when i first got him he was afraid of the stairs for about 1 week. he has a couple fears, but recently he has become deathly afraid of thunderstorms and fireworks. 1 year ago on the fourth of july, he hid under my bed the whole day. whenever theres a thunderstorm he huddles into a corner and starts shaking and drooling alot, or he hides under a bed. when theres a firework, he just hides under my bed and shakes. why was this so random? he never used to be this way.