The comet has some gravity. The lander bounced because the grappling and thruster malfunction, while it landed on shock absorbers.
I thought the gravity was too weak to help if it bounced. Your explanation would make sense, but it runs counter to what I thought I heard on one of the explanatory videos. I can only see too possibilities. One is gravity. The other is that the robot could “bounce” into the path of the comet, or into the path of another part of the comet as it rotated.
The gravitational acceleration of the comet is very small, calculated to be about 1.5mm/sec^2, so it would not take much of a bounce to send it back into space. . . and a very long time to bring it back down just by a fall. Rosetta IS in a free-fall orbit around it.
I seriously suspect the failure of the harpoons and thrusters occurred because of electrical discharges from the comet disabled them by frying their electronics. The team did say there were other electronic anomalies associated with the landing.