Why is it advised to turn the aileron away from quartering tail-wind during taxiing? This question is related to a single engined cessna tricycle gear airplane. e.g, C152/C172.

The theory states that if the quartering tail-wind is from the right, the aileron needs to be all the way to the left with elevator held down. Can someone explain the physics behind this?

I understand this when you have a quartering headwind from the right. In this case you set the aileron on the right thereby reducing the lift on the right wing and increasing the lift on the left wing. This helps keep the right wing pressed down. But somehow the picture is not clear for the quartering tailwind.

Another way to look at this is to look at the wind as a vector. For a quartering headwind, the wind can be thought of as a wind component from the front and a wind component from the right. The wind component from the front is not a problem and the wind component from the right is balanced by the ailerons as discussed earlier.

Taking a similar approach for the taiwind from the right, the wind component from behind is taken care of by the elevator. That leaves only the wind component from the right. So the same aileron action should be required. But the books state the complete opposite.
The mnemonic that I use is, "climb into headwind, dive away from tailwind." 

To address your question. 

When the wind is a quartering tailwind, the tailwind component tends to want to lift the tail of the airplane, which could cause a prop strike or the airplane to be flipped over if the wind is strong enough. So by having the elevator all the way down, the wind hitting the tail will push the tail down instead of lifting it up.

The quartering tailwind hitting the trailing edge of the up-wind wing can lift the wing, so by diving away from the tailwind, the upwind aileron will be in the down position, which will help hold that wing down. 

With regard to the physics of how this works, it has to do with the direction in which the wind is being deflected. When wind is coming from the front (headwind) an up aileron deflects the air up and pushes the wing down, but when the wind is coming from behind the wing, such as in a quartering tailwind while taxiing, the wind would catch an up aileron and lift the wing, so it is necessary to put the aileron down, so that the wind is deflected upwards thereby pushing that wing tip down.