For some reason, our brains are unable to tune out half a conversation. Researchers believe this is because we can't predict the speech pattern of a halfalogue the way we can with a monologue or two-way conversation -- making it harder to ignore.
R. Amant, T. Horton, and F. Ritter. CHI '04: Proceedings of the SIGCHI conference on Human factors in computing systems
, page 343-350. New York, NY, USA, ACM Press, (2004)