As an addendum, I looked on Wikipedia and there was no mention of a specific reason for using fireworks there on ID4.
Looking at the Fireworks entry did not yeild much information either for why they are used in ceremonies either.
I would venture a guess that since they were originally used in war as weapons that firing them off in a war victory was similar to firing a gun, which people do do when they win battles. So perhaps it went from war weapon to War victory celebration tool to general celebration tool. From there we end up where we are today.
Also I found a note of Vancouver, BC
Both fireworks and firecrackers are a popular tradition during Halloween in Vancouver, although apparently this is not the custom elsewhere in Canada. The only known fireworks used during All Hallows' Eve in the United States is the annual "Happy Hallowishes" show at Walt Disney World's Magic Kingdom "Mickey's Not-So-Scary Halloween Party" event, which began in 2005.