This is a psychological experience.
When I describe it in full, if you are of any age, you will almost certainly think "oh yes, that has happened to me."
For older listeners, the name Baader-Meinhof might be memorable as a name given to a group who liked to call themselves the 'Red Army Faction.'
The name Baader-Meinhof, after two notable members of the group, was given to it by journalists.
In the late sixties, all through the seventies, and even into the eighties, the Red Army Faction were responsible for a number of terrorist attacks in and around East and West Germany. One person's terrorist is another person's freedom fighter, but we will not get into that discussion.
This psychological phenomenon was given the name, after a man wrote a letter to a newspaper in 1994 pointing out that he had recently heard the name, Baader-Meinhof, and thereafter, seemed to hear or see it again and again.
Following that, many people wrote to the same newspaper making similar comments, about recently heard names, usually nouns, which were then noticed by them frequently.
The phenomenon was also given the name 'Frequency Illusion' in 2005.