It's not just raw skill but fatigue from the mental exertion of using a foreign language all day. You take classes in a foreign language all day and it's very hard to spend the evening hours also exerting yourself linguistically. I can empathize with the desire to just take it easy using your native tongue after a long day of keeping up with academics in a non native language.
I think this is the case. When a relative of mine went to Germany as an exchange student she very much intended (as the program advises) to make a lot of non-English-speaking friends. But speaking German was effort for her in a way that speaking English isn't, and after awhile she naturally gravitated toward other English speakers when she was ready to relax.
