Here the tables have turned. She left, swore she was done, and now she is back, and he is not having it. He teases her right away: "Que no estabas tan segura de irte" (weren't you so sure about leaving?), and "Doblaste las manitas" (you folded your little hands), a Mexican way of saying she gave in and came back begging. The title line, "¿A qué se debe?" (what's the reason for this?), is pure sarcasm.
He guesses the answer himself: "Será porque un cabrón por ahí te jugó mal" (could it be that some jerk out there did you wrong), and he makes it clear his arms are not a shelter anymore, because "Yo no soy ningún tapete para pisotear" (I'm no doormat to be walked all over). The bluntest line is "Si te hago falta es muy tu pedo, porque tú a mí no" (if you miss me, that's your problem, because I don't miss you). Tu pedo is crude Mexican slang for "your problem". He also mocks her old promise "nunca volveré" (I'll never come back) with "me cae que poco duró" (I swear that didn't last long).