Comment by PhantomPhreak

Bitcoins were sent by the Bitcoin community to an unspendable address (1CounterpartyXXXXXXXXXXXXXXXUWLpVr, which has no knowable private key), and the Counterparty protocol detects this and creates XCP in proportion to the destroyed bitcoins, crediting them to the sending address. (This was only for the month of January... don't send any bitcoins to that address now!)