Why is the exclamation mark called a "bang"?

Punctuation apparently got short names and nicknames in the printing industry when humans set the type, literally by putting small pieces of metal into frames. They needed to say "pass me a ..." and multi-syllable names like "exclamation point" didn't work as well for them as short and easily distinguishable ones.

Something similar happened in the early days of computers, when we had to type a lot of things and often tell each other what to type. Again, short names that didn't sound anything like each other were preferred to long ones. The Unix guys, who like to use words related to magic and wizardry whenever they could, re-animated many of these really old fashioned printer's type words and invented a few of their own. (Here's a list but there are many more you can find - just search for a few of the nicknames and you'll find pages with lists of them.)

As to bang, if you read the comics you'll see that certain words are always written with an exclamation mark, and Bang! is one of them. It gives more emphasis to the word, which is representing a sudden loud sound like the firing of a gun. (Things can fall over with a bang, but it's not the canonical example of the sound.) Printers also called it screamer, I guess because it could change an ordinary sentence like get out to something that was screamed like get out! Like all names, it doesn't need to "make sense" - why is a rose a rose and not a tulip? - it just needs to be memorable, which it is.

Found at "The pronunciation guide for UNIX"
LINK

! bang

comes from old card punch phenom where punching ! code made a loud noise; however, this pronunciation is used in the (non- computerized) publishing and typesetting industry in the U.S. too, so ...
Alternatively it could have come from comic books, where the words each character utters are shown in a "balloon" near that character's head. When one character shoots another, it is common to see a balloon pointing at the barrel of the gun to denote that the gun had been fired, not merely aimed. That balloon contained the word "!" -- hence, "!" == "Bang!"