France is a bit of a different story. The people in France who switched to English with me (and there were at least several handfuls) mostly did it to practice their English. English is far less common in France than in Montreal, where about half the population (I think) is anglophone. In Montreal, people will more usually switch because:
1) they're trying to be helpful
2) it's the policy of the company
3) it's just easier to switch back and forth
4) they're trying to show off

Not everyone does it for all of those reasons, but those, I think, are the common ones.