US threatens to impose sanctions against the EU if they drive to hard of a bargain with the Brexit. Chris Giancarlo, chairman of CFTC said the US will take countermeasures if Brussels demands that the so called 'clearing' of financial transactions using euro currency are made on EU soil. This would force part of it's financial sector to abandon London, and move to the EU.
London owes much of it's status as the world's financial center to the fact that most of the world's largest clearing offices have settled there. A forced move to the EU would cost the City 83 thousand jobs, according to a studies done by consultancy club EY in 2016.
Three quarters of the world's financial derivatives that are denominated in euros are currently being taken care of by London clearing offices. 850 billion euros flow through each day.
If a clearance agency gets into trouble, the consequences for the financial markets can be huge. This is why the ECB wants that clearing houses trading in euro derivates are settled within it's own sphere of superintendence. This means that they will be forced to move from London, when the Brexit becomes fact, or cease trading in euro derivates (not an option).
However, the US fear 'balkanization' in other words, fragmentation of the financial markets, when every country or monetary union decides to take care of it's own clearing. This would lead to higher overhead costs for financial institutions when trading stocks or derivates. Apart from that, they think that fragmentation will make monitoring the clearing offices become less effective.
The discussion is not new. In 2015, the British were put in their right by the EU Court of Justice, which ruled that clearing offices in London were still allowed to handle transactions in euros.
The US openly threatening with sanctions in support of the UK is new though.
http://www.volkskrant.nl/buitenland/vs-sancties-als-eu-londen-hard-aanpakt-na-brexit~a4494188/https://en.wikipedia.org/wiki/Clearing_(finance)
https://en.wikipedia.org/wiki/Commodity_Futures_Trading_Commission