Keep in mind: I am not a vocal coach. Although I believe strongly that what I have written is true, I don't have references for this. This may not all be technically, historically, or anatomically accurate, so take what I say with a grain of salt - especially if you're trying to learn this. Get an instructor or risk permanently damaging your voice.
After listening to it, the guy in the middle seems to be singing "dirty" the whole time. The general term for the opposite of a clean-sounding vocal is singing with distortion. Distortion can be introduced intentionally by the singer in a lot of different ways (some of which are more healthy than others). Most types of distortion involve obscuring the fundamental frequency to some degree.
This example sounds very intentional, thus it is being used for effect. In particular, right after the key change, the middle singer's distortion sounds very similar to certain types of rock/metal distortions: "I tried to hide but now you know it" sounds extra-aggressive. At other points in the song, the distortion sounds a bit more stylistically pop. I would guess that since that section involves singing with way more power and energy than the rest of the song, the singer chose to tap into even more extreme distortion.
The basic principle behind most types of distortion is that some extra noise must be introduced to interrupt or combine with the normal vibration of the vocal folds. Sometimes this is done with the false folds (I would wager that's what the man on the right is doing with that growl), sometimes it is the vocal cords themselves being allowed to drift out of sync (like vocal fry while also singing regularly). There may be other ways to do this.
Certain types of vocal distortion tend to happen naturally in a person's voice when they scream, shout, or yell at intense volumes - with so much power being forced through the vocal mechanism, a bit of a growl or a roar may come out. Other types happen when they attempt to speak while crying. Usually that involves some kind of discoordination of the vocal muscles producing the rasps and crackles of a broken-sounding voice. There are also some types that can be heard as a result of actual damage to the vocal cords; for example, smoking or drug habits can sometimes introduce a rougher quality to a person's normal voice.
One of the reasons that distortion is used in singing is to draw upon the conditioned human empathetic response. The rock/metal distortion types attempt to convey anger by using distortions that can be heard in a person experiencing rage in real life. The pop distortion often plays at imitating the breaking sound of a person in grief or loss. The blues, rock 'n roll, and R&B styles of distortions may be suggestive of the roughening effects of lifelong demanding labour on the voices of slaves in the United States. Of course, each singer has their own voice, and there is much overlap between all the different ways one can alter it.