My other reply is also about this, but if there's not a backstory here I think an out of the blue outrage could come from how this conversation is "traded" online.

This is a new idea for me, so bear with me as I'm hammering out the details and the language.

There are a lot of frank conversations online about race. These conversations are largely anonymous so people can spout controversial or offensive positions very easily. These conversations are also at will so people can walk away if things don't go their way, leading to a huge number of very heated but unresolved conflicts.

These conversations are all carried out by flesh and blood human beings, none of whom can completely detach themselves emotionally from the content. This creates the ideal conditions to breed the strongest, negative, emotional links to the most controversial and incendiary topics.

Once these links are in place, it can be very difficult for an individual to to be confronted with the topic to separate the emotional response built in other conversations from the future discussion about to take place.

That's the idea, I've been trying to work out any way.

But in this case I think something happened at the last pick up game:

...you take up space, steal the ball, don’t pass,...

...non passing the ball, stealing the ball from beginners,...

These are so specific that it seems like he's been stewing on them for a while.