The default channel definitions is likely where you should start:
Note: (Other) and Other Advertising are not the same. (Other) is where traffic gets tucked when it fails to fit into the ones listed at the link above.
And look inside your Analytics, because default channel criteria are editable.
There are many ways Google can identify the Source, Campaign, and Medium, one way is to pluck them from inbound URL requests,
So I could enter [yourdomain.com
And these values would post.
Analytics needs Source, Medium and Campaign values to be valid, and it needs to fit them as the link here says.
So if somebody builds a sloppy crawler, and it cranks out weird utm values, you'll see weird values.
Dig into where the traffic lands, see if you tell if someone or something is aiming at certain pages.
You might even have tools that check your own website, and they issue weird params in the URL or encode strange chars, that Analytics will misunderstand.