Captives held by the victorious Parthians would no doubt have been a tradable commodity just like silk. As the intermediaries on the land route (also called Silk Road), they certainly had that ability. Yon Cassius, he of the “lean and hungry look”, was respected for having brought 10,000 Roman soldiers back to Roman controlled Syria from Crassus’ deadly campaign. This was only a small portion of those who had marched east. In fact it was Caesar’s “ambition” to go fight the Parthians again to reclaim Romes honor and avenge Crassus’ death that motivated Cassius to the assassination of Casear. Cassius knew from first hand experience just how deadly the Parthians were in warfare. I believe this was in the area of current Afghanistan. Those who fail to learn the lessons of history are likely to repeat them. At least Rome did not repeat that mistake.
Huns are often considered proto Mongols. The first large scale Hun raids occurred in the 4th century. It’s not out of the question that these skeletons were the result of the slave trade, or even Hun warriors who signed on with Rome. The classic infantry-heavy Roman formations were changing in response to the appearance of bow-armed mounted (vs chariot) cavalry, and Rome would have been recruiting mercenaries and outcasts from enemy formations to figure out how to meet the new challenge. The Huns are often considered proto Mongol tribes that fled Chinese military pressure for regions less accustomed to their military tactics.