Apparently, "crew in production meetings were getting confused about what scene was in what film."
Looking back now, after three Avengers movies have come out, it's almost impossible for someone who isn't a diehard fan to tell what happened in what movie. They kind of just blur together as on giant movie. Apparently, this happens to the people who film them too. The Russo brothers, Joe and Anthony, explained at a Q&A last night why chose not to film Avengers: Infinity War and Avengers 4 concurrently: it would be way too complicated.
Although the event wasn't filmed, a film blogger named Peter Sciretta, was at the event and he tweeted out details as the came. Apparently, "they originally planned to shoot both films at the same time but the stories got too complex" and they didn't want to get the tones of the movies mixed up, because they're "very different films." Apparently they made the decision to shoot the movies separately only a few months before filming began, as people were getting too confused about what was what. The Russo brothers also seem to have a chill sense of humour: