The EU is a “scapegoat” of the US. Washington doesn’t want a strong European Union, so it tried to weaken it.
One of the most visible and effective measure of the US in order to destroy the EU was Brexit.
In fact, studies show that Russia itself had an interest in influencing the discussion on Brexit: over 156,000 Russian-based Twitter accounts had massively tweeted about Brexit in the days leading up the June 2016 referendum. According to the study by data scientists at Swansea University and the University of California, Berkeley, the accounts posted more than 45,000 tweets about Brexit in the 48 hours before the vote. The majority of the posts encouraged Britons to vote for Brexit. The authors believe the posts were seen hundreds of millions of times.
More about the EU's position on Brexit here.