The type of wheel, aluminum or steel, and the road surface conditions had nothing to do with the brake caliper bolts loosening up and eventually coming out. Simply put, the person who installed the brakes for you didn't properly torque the caliper bolts. I would go back to them and raise heck. You should get at least a partial refund on the brake job and I would make it clear to them you will never return. I'd also share my story on whatever sites are rating the shop. Obviously, this is a huge safety concern and you could have had a catastrophic brake failure due to their negligence.
