[jjsheridan5]

That still leaves my question unanswered: why would it bounce (as opposed to deflect off into space)? The harpoon and the thrusters were the only systems that seemed to allow the robot to return to the surface after impact, should the robot fail to immediately anchor itself in, and neither are working. The screws would help the craft stick the landing, but wouldn’t seem to be of any help if it bounced.

[HandyDandy]

The comet has some gravity. The lander bounced because the grappling and thruster malfunction, while it landed on shock absorbers.

[Cboldt]

It would bounce because the landing legs have resilience, the lander is "bouncy" like a spring.

CGI animation of landing

On landing, the legs damp out most of the kinetic energy to reduce the chance of bouncing, and they can rotate, lift or tilt to return the lander to an upright position.

The Rosetta Lander

Because of the comet's extremely low gravity, a landing gear will absorb the small forces occurring during landing while ice screws in the probe's feet and a harpoon system will lock the probe to the surface. At the same time a thruster on top of the lander will push it down to counteract the impulse of the harpoon imparted in the opposite direction.

[Cboldt]

Still learning about the landing craft ...

There will be two harpoons to anchor it to the surface so that it can be reeled in like a fish on a line. There are also ice screws in each foot, which can be rotated to help to secure the spacecraft on the surface. The lander is also designed to stay upright on a slope of up to 30 degrees.

From Rosetta FAQ