I had the same experience when I was doing an internship in Germany. One particular coworker kept speaking English to me, despite everyone else speaking German to me. At this point, I had lived there for a year already and spoke German pretty well, so comprehension definitely wasn't an issue.

I think he was either used to speaking English with non-Germans or just liked having the opportunity to speak English.

I just kept replying in German and he eventually switched to speaking German to me.