In August an Iranian official was arrested in central Afghanistan (near Herat) and accused of illegally recruiting Afghan Shia to serve as Iranian mercenaries fighting in Syria. While Afghanistan wants good relations with Iran since 2015 the Afghan government has been quietly trying to get Iran to back off on its recruitment of Afghan Shia to fight in Syria. Iran tried to be more discreet in Afghanistan but did not stop.
This recruiting began among the three million Afghan refugees (from the fighting in the 1980s and 90s) still in Iran. Many of these were there illegally, some as a result of efforts by Iran to get the Afghans to go home. For potential recruits the main draw is money (up to a thousand dollars a month) and permission to legally settle in Iran (along with his family) if they survive. Over five percent of these volunteers were killed in Syria and Iran compensated their families, if only to encourage more recruits to go defend their Shia faith (and Iranian interests). Some training was provided in Iran before the Afghans were sent to Syria to join the Fatemiyoun Brigade, which Iran had organized in late 2014 for Afghan Shia there to defend the Shia Assad government of Syria that has been fighting a rebellion (mostly by the Sunni majority of Syria) since 2011. In Afghanistan the Taliban (and Sunni Islamic terrorists in general) consider Shia heretics and often kill or kidnap them.
Iran officially denies this recruiting was ever going on but there was growing evidence that it exists and is believed to have sent over 5,000 Afghan Shia (including illegal immigrants living in Iran) to Syria so far. While many of these Afghans did not survive the experience it was the families of those recruited inside Afghanistan who complained to the Afghan government and that led to the recent arrest of the Iranian recruiter.
Iran has long supported the Shia in Afghanistan. Only 15 percent of Afghans are Shia and these Shia are a particular target for Sunni Islamic terrorists (like the Taliban). Most of these Shia are the Hazara, who are ten percent of the population and the descendants of the hated Mongols who conducted several brutal invasions during the 13th and 14th centuries and destroyed more of the country and its population than any other conquerors. For centuries Hazara have suffered a lot of discrimination and actual violence in Afghanistan.