Money chases the sun. At the end of trading in New York you make a swap with a company in Japan that at the end of trading in Japan does the same with London then back to New York. Any money not swapped doesn't make money overnight.
This is called the overnight repo market. But if you don't get your money back in the morning, you've got a problem.
http://www.fintools.com/docs/Warren%20Buffet%20on%20Derivati... is nearly 20 years old, but Warren Buffet's criticism of how we account for the value of derivatives is as true today as it was then.
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."