So, how is this related to the Younger Dryas?

Well, the tusks being picked up will be those closest to the top at first - which are the ones most involved in the Y-D event - from when they went extinct. These are the ones that are... snapped in half from mud-slides, peppered with comet debris and radioactive isotopes. Picking out the tusks would also likely disturb and destroy the ground around - including the remaining skeleton and any evidence of how it died.

Basically, it's disturbing the integrity of the whole carcass, the evidence that will help determine what happened 12,000 years ago to end the last ice age, is being conveniently picked off.