Why is "coon" a word?

The word formation process that yielded the word coon is called (fore-)clipping:

raccoon > coon

Other examples of fore-clipping include: bot (robot), chute (parachute), roach (cockroach), coon (raccoon), gator (alligator), phone (telephone), pike (turnpike), varsity (university), net (Internet).

For some of these examples, the clipping simply reverts a previous compounding: notably, it preserves existing word stems.

But not for raccoon, which comes from Algonquian arahkun: there does not seem to be anything to revert, and the word simply becomes one syllable shorter.

Similarly, bot does not preserve word stems: robot derives from robota (forced labor), which in turn (reportedly) derives from PIE *orbh-.

I wonder whether there is a driving force that helped establish these words as lexical units (such as, a strong preference for one-syllable words), and whether there is a pattern for how words such as these can be born, or whether they emerged and caught on by mere chance.

Native speakers often know nothing about the derivation of words they use. Who knew that raccoon was an Algonquin word?

Daily or otherwise frequent encounters with the object referred to by the noun probably has a lot to do with the clipping.

A person who has nothing to do with robots (who doesn't make them, who doesn't read sci-fi books about them, who doesn't spend any time on the internet to speak of)—an elderly dairy farmer in Iowa, perhaps— is not likely to refer to a robot as a bot. And an investment banker on Wall Street, say, is far less likely to use the word gator than someone living on the bayou.

These clipped forms can eventually make their way into the general vocabulary in a number of ways. There are lots of TV shows, for example, about alligators these days.