[alexpotato]

Great question! Realized I was mixing different company sizes in the original post.

This would most likely be hitting smaller to medium sizes firms (e.g. Hedge Funds) that are holding a larger position in a security. The small/med firms don't "self clear" so they tend to depend on a larger firm to do that for them.

I should also point out: I used the example of one big position but it could also be a bunch of medium sized positions and then the PB says "Hmmm, we don't agree with 1 or more of those positions so we are stopping trading with you on all of them."