Why would anyone do this to a coin

Possibly a test cut by a non US banker/owner to determine if full silver. This was done quite often.

 

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The only way to figure out if it is solid silver all the way through is to cut it open and look inside!

 

No, a cracked planchet wouldn't have such a straight cut. A cracked planchet is usually going to have a more jagged appearance.

I wouldn't expect that on such a low value coin. That may be possible on a silver dollar, but not a trime.

The trime also was one of the first intentionally underweight US coins. It's value was higher than it's silver content, and so a banker's mark on this coin makes even less sense.