My history lobe is a bit rusty but after emancipation, black folks grew a lot of watermelon because it was easy and cheap to grow and it sold well. It was a symbol of the time as well as former slaves reaching financial independence. As usual, shitheads will turn any symbol against it's people as a tool of oppression (see DEI as a recent example) so watermelon was associated with black people in the worst way. Now the racism has died down a lot but to this day some people will say (and think) that black people love watermelon and think of it as a poverty food.

It goes way deeper but that should be the semi-accurate gist.