It seems like especially in Nordic Europe, the vast majority of people speak English as a second language. How did this come to be?
you might be interested in responses from this thread
but it's just more anecdotal info; maybe try x-posting to /r/AskSocialScience
Because we have little choice. In my home country, we start learning English from 6-7 years old, and almost all TV programmes and films are in English. Nothing is dubbed. Plus, English-language music is more popular than music in the native language.
You also need English to use most of the Internet.