The USA and Canada have agreed to tell each other before deporting citizens to third countries.
In theory, that's to not repeat the horrific story of Maher Arar, the Canadian programmer, born in Syria, who, while changing planes in New York on a flight from Europe to Montreal, was grabbed by U.S. agents, grilled and then deported to Syria, allegedly so the Syrians could torture him in ways the U.S. could not.
Arar wasn't even trying to get into the USA. He had been investigated on some fairly weak connections and cleared, but his name got on the lists. The Syrians tortured him for suspicion of membership in a Muslim organization opposed (like the USA is opposed) to the Syrian regime while he was a teenager in Syria.
Instead of being deported to Canada, his nation of citizenship, he went to Syria.
This is a particularly nasty story which you should know about if you are a U.S. citizen. Arar was cleared, but the hard truth is the USA shouldn't be setting even the guilty up for torture, let alone the innocent.