Random car (Latifi) crashes. Safety car comes out for safety purposes. Safety car means all cars are grouped together. This takes away the lead Hamilton had built up. That happens quite often. As it happens, the guy in second (Verstappen) pitted for fresh tires, which means he has more grip, can go faster. Car in lead (Hamilton) had old tires too, but chose not to pit. Then shortly before the end the race director let's all the cars between 1 & 2 (all were one lap behind) unlap themselves so they wouldn't interfere with the race for the win once the safety car pitted and racing could resume.
That's controversial because usually either all cars or no cars are allowed to unlap themselves before racing. Mercedes say that's the rules, Red Bull says that's not what the rules say. Race director states all teams have agreed in the past that it was highly desirable for the race to end in a 'green' condition' rather than behind a safety car. Letting all cars go past would likely have led to another lap behind the safety car, and there was only that one lap left.
Extra frustration from everyone because the race win also means the championship win.
