Answer by taking a quote from the following article:

Secretariat (March 30, 1970 - October 4, 1989) was an American Thoroughbred racehorse who, in 1973, became the first Triple Crown winner in 25 years. His record-breaking win in the Belmont Stakes, where he left the field 31 lengths behind him, is widely regarded as one of the greatest races of all time. During his racing career, he won five Eclipse Awards, including Horse of the Year honors at ages two and three. He was elected to the National Museum of Racing and Hall of Fame in 1974.

The syndication deal for Secretariat precluded the horse racing past age three. Accordingly, Secretariat's last race was against older horses in the Canadian International Stakes over one and five-eighths miles on the turf at Woodbine Racetrack in Toronto, Ontario, Canada on October 28, 1973. The race was chosen in part because of long-time ties between E.P. Taylor and the Chenery family, and partly to honor Secretariat's Canadian connections, Laurin and Turcotte. Unfortunately, Turcotte missed the race with a five-day suspension: Eddie Maple got the mount.  The day of the race was cold, windy and wet, but the Marshall turf course was firm. Despite the weather, some 35,000 people turned out to greet Secretariat in a "virtual hysteria" that Secretariat seemed not to notice. His biggest opponents were Kennedy Road, who he had already beaten in the Marlboro Cup, and Big Spruce, who had finished third in the Man o' War. Kennedy Road went to the early lead, while Secretariat moved to second after breaking from an outside post. On the backstretch, Secretariat made his move and forged to the lead. "Snorting steam in the raw twilight", he rounded the far turn with a 12-length lead before gearing down in the final furlong, ultimately winning by  6 1/2 lengths. Once again, many winning tickets went uncashed by souvenir hunters.  After the race, Secretariat was brought to Aqueduct Racetrack where he was paraded with Turcotte dressed in the Meadow silks before a crowd of 32,990 in his final public appearance. "It's a sad day, and yet it's a great day", said Laurin. "I certainly wish he could run as a 4-year-old. He's a great horse and he loves to run."  Altogether, Secretariat won 16 of his 21 career races, with three seconds and one third, and total earnings of $1,316,808.  For 1973, Secretariat was again named Horse of the Year, and won Eclipse Awards as the American Champion Three-Year-Old Male Horse and the American Champion Male Turf Horse.

what happened there?
The race was chosen in part because of long-time ties between E.P. Taylor and the Chenery family, and partly to honor Secretariat's Canadian connections,