Have you ever started a sentence in one language and finished it in another? Or squeezed another language in to make three?
When you work daily with people of different linguistic backgrounds, say, to take an example from my own experience, French, English, Spanish, Gujerati, Russian there are ample opportunities for this sort of language-zapping.
But why should you do this? Perhaps because in a given situation a particular expression in a particular language seems to better describe what is happening or encapsulates a concept better?
Perhaps because you need to show off your extensive linguistic knowledge and this seems to be a good way to do it? I'm guilty of this sometimes.
I'm sure people who have been brought up in truly multilingual circumstances do this all the time. It feels a bit stranger for those of us who have had to acquire new languages through hard work.