Background check after quitting?

I’m not sure if this is the correct place to post this: Last year, I quit a job because they went back on their end of the job contract that we signed when they hired me. I now found out that on my last day, they conducted a background check on me. I never consented to this. Is this normal and does anyone know why they would do this? I can understand conducting one when hiring me, but why conduct another one on my last day?