Here is the Cat track wear simulator program that they designed to illustrate exactly how track systems work, and what causes them to wear when in poor condition or not assembled correctly.  The real answer to the question of what wears when the track chains are on backwards (or even when traveling in reverse) starts at around the 3:00 mark of the video, but the whole thing is well worth a watch because it does a great job of explaining all aspects of undercarriage design and operation.

Interesting video, thank you for posting that. Mike