It could maybe also be affected by perception of the role of one's own culture, and whether they are within that culture or have migrated elsewhere

Spanish-speakers who feel that their home country is somewhat isolated, restrictive, cut off from global culture could feel they are more involved in the world, so more 'outgoing' when speaking in English, rather than the language of their narrower home language community.

Lots of very interesting points raised in this article, Liz, and plenty of links to dig into!