It is not, this is a totally normal thing almost everybody does. Listening to songs create mental pathways and those can be triggered by hearing the words in another context. Especially for lyrics/songs that are designed to be easy to remember/catchy.
It only becomes a sign of mental disorder when you cannot prevent yourself from singing the song upon hearing the lyrics. Just hearing the lyrics and even singing the song is absolutely normal. Even when it is related to a mental disorder, this kind of thing isn't indicative of a specific disorder. It can be an indicator of a form of autism, ADHD, a compulsive disorder or PTSD. Even someone who is normally healthy but is under a lot of stress could display this symptom.
This is why self diagnosis is so dangerous, just because you display some symptom that could normally be associated with some disorder absolutely does not mean you actually have that disorder. If it even is a symptom to start with.