In the 1850s, why did many runaway slaves go to Canada?
The Fugitive Slave Act allowed slave hunters to pursue runaway slaves into the Northern States and transport them back to the South. Local Law enforcement was obliged to help the pursuers.
The "Underground Railway" existed primarily though Detroit into Southern Ontario or "Upper Canada" as it was then called. It was neither "underground" nor a "railway". It was just a secretive way of referring to the route. The Fugitive Slave Act (1850) was part of the ongoing struggle between Anti and Pro Slavery forces that was to lead to the Civil War.
The Act allowed slave hunters to pursue slaves into the Northern States. Once there the slave hunters might deem any black man who crossed their path as a runaway slave. There was a cash reward for the return of slaves.
The route was well used since the 1780s and saw an increase after 1850.