I only have second hand accounts but I've heard the Instagram CEO just hated having employees in London (/ outside of the US) and therefore started with closing Instagram positions there. Then the rest is following.

And to be clear I'm not sure a UK employee is that much cheaper than a US one. The salary is not THAT far off between the two, especially when converting from GBP to USD, and employers have a lot more social charges to pay on top of salaries in the UK and Europe.

If you add the cost of collaborating across very spread timezones, I really don't think hiring outside of the US is that much cheaper.