It has it's roots in African-American culture from during slavery, but it's a Welsh and Irish tradition that had something to do with the marriages not being recognized by the state. The enslaved people adopted it because their marriages were also not legally recognized.
There was this one ladies wedding where the soon to be mother in law wanted to honor her blackness by having jumping the broom at the wedding - fiancé/groom and his mom were very very white. Except the bride was, like, Nigerian and didn't have any connection to it as a tradition. And the mom just wouldn't get it. I think she ended up boycotting the wedding with her family or something at the end??? And kept insisting that they have jumping the broom at the ceremony