Let me correct you here. Zainabiyoun was raised from the areas where there were lot of Shia youth was suffering at the hands of takfiris/wahabbis already. Taliban and takfiri affiliates have been doing unspeakable horrific crime against innocent men, women and children. The route to Parachinar had been closed for months or even years.
Now comes the Syrian civil war of 2011. A imperfect but still somewhat stable Govt. of Bashar was destabilized by the Gulfies and Turks with full US involvement. Saudis and Qataris brought about their favorite Salafi head choppers from all around the world, including from Pak tribal regions that no one talks about. These were/are the human animals who happily decapitate and even consummate their opponent. What reference from Uhud I am searching for here.
Anyways, Iran also had to protect their Axis of resistance and interests. The protection of the shrine of the Grand daughter of the Prophet (SAWW) in Damascus only helped justify their involvement even more. We saw what Salafist/AQ did to the shrines of Shia Imams in Samara in 2006. The one single event that lead to a full blown civil war in Iraq with hundred of thousands killed in suicide bombings and the reprisal raids.
So now given this, which Shia would not have sympathies with any Govt in the world which takes a clear stance against these takfiris and protect their holy sites.
If IRGC recruited Shia youth from the sectarian strife affected regions, it only more understandable. I have not said it was justified for the people to heed to the recruitment call but understandable.
But an even more important question, our intel has usually very good understanding of the ground situation in such areas. Or at least this is the nominal assumption. Why they did not pre-empt and stopped/dissuaded people from joining groups like Zainabiyoun while the whole thing was in its infancy. Even more basic question, why the situation in places like Parachinar was allowed to fester for so long that people saw a foreign Jehad a viable option.
So thing are not as simple that why Shias went to fight Sunnis. Look into both sides of the arguments.