It has to do with degrees of freedom and vibration dampening. A sandbag generally will not restrict movement (if it's not packed too tight), so when the primer fires and the powder ignites, and the bullet slams into the lands, it all sends a vibration into the rest of the firearm. The vibration will travel much faster than the bullet. The stiffer the rifle, the less dampening effect and the faster the vibrations will travel and reverberate back and forth through the gun.
The rifle has inertia, so it takes energy to move it's center of gravity (CG), generally the heavier it is, the more energy it takes to move the CG (moment arm is also a consideration).
When the rifle is sitting on a soft sandbag and the vibration starts down the rifle, the sand will displace as the gun is free to move downward when a vibration pulse hits the part of the rifle sitting on the sandbag, i.e. the gun would rather not move it's center of gravity when it's easier to move the sand(conservation of momentum).
When a bipod is used, the downward freedom of movement is restricted, so when the pulse hits the part of the rifle sitting on the bipod, the bipod does move as easily downward, and the pulse ends up kicking the CG upward. Hence the dreaded bipod bounce - it generally follows that the stiffer the rest, the more bounce you will get (springy bipods complicate this point).
In the end, bipods are best on soft ground, not on hard benches as the soft ground will absord the pulse like the sandbag.
Other factors that aggravate the bounce is spring in the stock, so stiff is ok, but the dampening effect of the rest is what is really important.
Jay
