I’m betting it’s the one that says
The Secretary has determined that the applicant’s activities abroad are causing or likely to cause serious damage to the national security and foreign policy of the US.
Manafort has been diligently working with Russia to help them subjugate Ukraine for decades. He was instrumental in rigging the Ukrainian election that put the Russian backed candidate Yanukovych into power, and jailing and persecuting his opponent afterwards. He had a huge tacky home in Ukraine with gold plated faucets and a 100 car garage. His former home is now a museum.
Most of the US attention on Manafort centers around his work with Trump, but that’s probably the least evil thing he’s ever done professionally - and there is credible speculation that he volunteered to work with Trump in order to pay off his debt to Russian oligarchs.
The Senate report on Russian interference, which is way longer and way more damning than the Mueller Report, includes the outlines of one Manafort effort, a very specific plot to carve out an Autonomous Republic of Donbas in Southeastern Ukraine, a plan that including getting Trump to wink and look the other way. This was going on while Trump was in office.
Details and more can be found here.
So, I’m not surprised they took his passport, and I’m REALLY not surprised that he was heading to Dubai, because apparently all the oligarchs are fleeing there to escape the consequences of the recent sanctions. And if his ultimate intent is to flee to Russia, it won’t be the first time, he fled Ukraine for Russia in 2014.