The short of it is that I fought alongside YPG against ISIS nearly a decade ago, and some boneheads at CBP/DHS flagged my passport. Ever since they've had a craw up their ass to try to catch me doing something, and problematic detentions whenever I re-enter the US (as US citizen with valid passport). The last time was the worst, where I got some particularly corrupt individuals who were willing to lie to get a warrant. Of course nothing was found, because in reality I'm not doing bad/illegal things overseas nor at the border.