These new interests can range from listening to new types of music, taking up new hobbies or watching movies. These new interests correspond with some person they have met and have an interest in them. Sudden habitual changes are always suspicious because people don't tend to change overnight. Habitual changes often go hand in hand with changes in mood, like for instance, sudden bursts of happiness or tears.
You might remember that when you first started dating you always tried to compliment each other's tastes in music, movies, and books. Your mutual tastes were influenced, you started listening to the music that they listened to and they started watching the movies and series that you liked to watch. So the change of interests is the same as that, only with another person.