Sorry to hear about this and glad no one was behind you this could have been A LOT worse as this comes crashing into a cars windshield.....
I always inspect my rack every so often. The rubber feet compress over time and the hooks may need tightening.
Also I have had a board slide back on me to the point it nearly slid out of the rack. Usually happens when my board is solo or just 2 boards. I try to keep a binding to the front of the rack to stop this and place them upside down as well.
-I'm Slyder and I suffer from "Gummer Syndrome"