It's likely transmitted by a small mammal, they aren't sure which one.
It's pretty hard to build a rat proof fence, especially across hundreds or thousands of miles, and it is an "outbreak" concern more than anything, it doesn't infect many people:
https://www.cdc.gov/poxvirus/monkeypox/about.html
The reason they are trying to trace it is that every human infection is an opportunity for an adapted, transmissible virus to emerge.