Interesting that this has happened so soon after relocating to the UK (2014) from Australia. I wonder if they plan to stay there or move the US as well?
According to statements made by the founder[1], the move to London was designed to make them more integrated into the global economy ahead of their IPO. That tells me that they aren't moving again any time soon.
[1] http://www.afr.com/technology/atlassians-farquhar-justifies-...
They are primarily based in Sydney, they might be the biggest importers of developers in Australia actually ;) but the SF office is large too, they hold a few usergroups in it, as well as the Atlassian Summit which just ended (3-5 Nov). https://summit.atlassian.com . No wonder you believed they were SF based.