The correct answer to this open question is the following.
In the 1850s, why did Harriet feel compelled to escort her escaped slaves all the way to St. Cathrine? Why, for example, didn't she stop in Albany?
Because if they stopped at Albany, New York, Northern people had the obligation -by law- to capture them and returned to their former owners. That is why Harriet had to take them out of the United States territory.
This happened because the Fugitive Slave Act passed by the US Congress in 1850, ordered people in the North to never protect runaway slaves. So this meant that the North was no more a secure place to keep slaves.
That explains why Harriet Tubman went up north to the Canadian territory because Canada indeed was a safe land for fugitive black slaves that scaped the southern plantations.