A son de qué is an idiomatic phrase that translates to "on account of what" or "for what reason." It's a more colloquial and expressive way of asking "why?" or "what's the point?"
In the song, Feid uses this phrase to express his confusion and frustration after his ex-partner suddenly left him: "¿A son de qué te fuiste? ¿Cuál fue la razón?" (For what reason did you leave? What was the reason?). It's a unique and engaging phrase that adds a touch of authentic street Spanish to the lyrics.
Feid spends this whole song arguing with someone who stopped answering. He opens flat out with Estoy cansado de aparentar (I'm tired of pretending), and the pretending in question is the story the two of them agreed on: that it was casual. Sé que no fuimos nada (I know we were nothing), he admits, and then takes it straight back with Pero fuimos first class (but we were first class). The detail that gives him away is ¿Por qué me tienes bloqueao'? (Why do you have me blocked?). That is not a philosophical question. That is a man looking at his phone.
The title comes from the chorus, ¿A son de qué te fuiste? ¿Cuál fue la razón? (What did you leave for? What was the reason?), which is the Colombian way of asking why something happened with no warning at all. What he actually wants is smaller than an explanation: No quisiera verte con alguien más (I wouldn't want to see you with someone else), then Que no sea yo, que no sea yo (anyone but me, anyone but me). By the second verse the two of them have been demoted to Ahora somos dos panas más (now we're just two more buddies), which is about the bleakest thing he could think to call it.