Hackers. Massive uses a trusted client network for the Division. So everybody and their mothers are just editing their client files and have free reign to do pretty much whatever they want in game. It really only happens on the PC version so outside of videos I haven't seen it first hand.
On the consoles it's just exploits people use and really it's not that many people. Right after 1.1 was released I would see a few super geared people in the DZ steamrolling, but not much anymore. What pisses me off is when I match make and get some GS203+ people and quickly realize that A) Their gear actually sucks something awful and B) They have no clue how to actually play the game cause they've exploited their gear and are worthless on missions or incursions.