Car rolled backward and almost hit another car

I was at a stoplight (red light) sloped uphill last night. The car was in drive and I had my foot on the brake. All of a sudden the car started rolling backwards!! I tried slamming on the brakes but nothing happened and the car kept rolling back. The car behind be backed up as I pulled the e-brake which took a few seconds to kick in.

I don't get it. I then gave the car some gas, with the e-brake still on and then it started driving fine like nothing had happened. The engine never shut down.

It was really scary and I have no idea what caused it to happen. My tranny has shown no sighn of slipping and the girl who sold me the car said she recently had a $500 brake job done. I just don't get it or know what to do. I guess I just hope it never happens again?? (The car went on to have no problems the rest of the way home, but I don't feel all that safe now)

Would appreciate any input. Thanks!