The Guardian have had a large focus to try to pick up US readership for a while (e.g. They launched CIF America).

The NSA coverage has probably helped them in that mission, in fact it is probably motivating a lot of their NSA coverage, the UK "switched off" over the issue almost day one.

Their switch to a .com domain, and expansion of US-focused coverage was well-timed to coincide with these revelations.