It's down to the mental fragility/insecurity that stems from the intrinsic knowledge the caucasian has, of his inferiority compared to the black man, in physicality and athleticism... to some it can be seen as (conscious or unconscious) racism, you decide.
Athletes, in general, are looked at with contempt by the public at large. They're seen as having gotten to their position primarily due to winning the genetic lottery, rather than perseverance and determination. To lesser extent, I could understand that view in basketball due to the huge disparity in height and size compared to an average joe, but certainly not in football where majority of the players are below 6'2 and don't have an abundant musculature, as it isn't a prerequisite in the sport.
Combine that, with the fact that everyone (particularly in the western hemisphere) have been inundated with the trans-Atlantic slave narrative... so Pogba's extreme case boils down to outward appearance; he's black and flamboyant. A flamboyant athlete will naturally be considered confident/self-assured, and someone who is so, will be considered as outspoken and rebellious (or potentially). If someone's seen as outspoken and rebellious, they'll be seen as someone willing to defend themselves physically or maybe even initiate it. Fighting an athlete is then a frightening thought, hence the reason why "thug" is sometimes used on social media for players like Neymar, Pogba, Lingard, Balotelli, Memphis Depay etc and never for Ozil, Bendtner, Ibrahimovic, Suarez etc.
When a flamboyant black athlete then does something preserved to be egregious, that's when those deep-seated feelings bear fruit in media/public perception and vitriol so being flamboyant/confident then turns into being cocky, egotistical and arrogant. An example being boxers like Floyyd Mayweather and Jack Johnson (King Kong was created to represent him and his love for white women btw).
Unlike the boxers, Pogba has a clean-cut image, and his respected and well-liked by players, and staff at United, Juve and France...still doesn't matter. Got opp fans booing him, since when did they care how United (the big bad bully) gets treated by it's player. We know what it is.
Frankly when Pogba joined the club, promises would have been made to him i.e play in his preferred position and role, ambitious decisions-making;
major trophies aspirations, squad improvement and recruitment of elite players of his ilk. Clearly those promises were long broken and in a field with a finite period of earning potential and achievement, that has to cause disillusion and the desire to be elsewhere. This is down to having a healthy evaluation and outlook but a black athlete of Pogba's caliber is supposed to be superhuman; immune from the effects of detrimental and handicap situations.
Conversely take N'golo Kante, his game is more so predicated on athleticism than technique, yet he's regularly showered with media/public praise. This stems from his shy, demure outward appearance.. essentially making him non-threatening.
I've yet to hear Kante criticised for a poor game or even a poor half, apparently he's flawless.. has a poor WC final, taken off early in the 2nd half, Pogba plays well, scores (it's all brushed aside, imagine if the roles were reversed). Kante's role is to shut down attacks through the middle but Chelsea midfield was a sieve regularly the last prior 2 seasons, particularly from counter-attacks but that was presented as solely the fault of the defence and Jorginho.
I'm of the belief that Kante, maybe due to injuries, as being on a decline the past 2 seasons. Still a good player nonetheless but he's still viewed as he was in his prime and in the future will placed in the class of Keane, Viera, Scholes, Yaya Toure, Gerrard, David Silva and Fabregas (underrated Prem great) even though his prime only lasted 3 seasons.
Jack Grealish, clearly flamboyant in both appearance and style-of-play whilst on the pitch, gotten into plenty law-breaking incidents just last year alone, yet the media's clamour is the primary reason he's starting for England... has the complexion for the protection. Southgate would prefer not to select him 'cause he views him as cocky/arrogant, having briefly played with him, he clears believes that his off-field behaviour reflects this.
When it's extreme, it's certainly down to both being black and the outward appearance. A more general example being, Rio Ferdinand booed by England fans, for supporting his brother in the John Terry racism case. Ferdinand had always played well for England but once you're seen as the "uppity negro" trying to challenge or disagree with a revered caucasian or an establishment (media/governing body/club) that's when... shit hits the fan.
but we got it carajo