I think it's the "dual standards" that people feel exist in how these events are Policed and reported. So for example -
Black rioters - Race not mentioned in media
White rioters - Far Right scum etc
Black rioters - Police stand back so they don't inflame the situation and no-one gets arrested
White rioters - Police crack some skulls, dozens in jail
Of course whether any of this is true very much depends on each individual case, but people mostly like to apply "generalisms" when they see the same things so often and those examples above are the sort of "general" thoughts likely held by far more people than would admit to it in public for fear of being called racist etc.