The Baader-Meinhof phenomenon occurs when a person, after having learned some (usually obscure) fact, word, phrase, or other item for the first time, encounters that item again, perhaps several times, shortly after having learned it.
There are several theories about the psychological explanation of the phenomenon, including a popular one that cites its primary cause as being the recency effect, in which the human brain has a bias that lends increased prominence to new or recently acquired information.
The Baader-Meinhof phenomenon is a form of synchronicity. A Jungian explanation is that the person learns the new phrase as part of a collective consciousness, which is also active in others. The concepts which float to the surface of the collective consciousness manifest themselves in different people at about the same time, leading to this effect.