WINDSOR, Ontario, Sept. 20 — Fleeing stepped-up sweeps by the American authorities, illegal immigrants to the United States, mostly Mexican, are arriving in growing numbers at the foot of the bridge in this Canadian border town seeking refugee status. Still more immigrants, mostly Mexicans living illegally in Florida, have begun trying to make their way past America’s northern border at other locations, the majority of them flying into the airport in Toronto, Canadian officials said Thursday. The arrivals here began suddenly three weeks ago, just a family or two at first, fueled by the notion — largely unfounded, the authorities...