Why did many african americans who had never been enslaved flee to canada?

A lot of them were fleeing with family.  They were helping others escape that were slaves.  They were being forced out, at times, because even though they weren't slaves, they were mistreated. They claim that slavery, at the time, was to protect African Americans.  They fed them, housed them, etc.  So supposedly it was because they were no longer safe. Also, back then, you could become a slave, unless you had your papers.  So some feared becoming slaves.