How does saddle nose happen after rhinoplasty if only thing that was done was rib cartilage was placed over bridge?

It would be very unusual to develop a saddle nose after rhinoplasty if "all that was done" was the placement of rib cartilage on the dorsum. The most common cause of a saddle nose is deficiency in the underneath structures of the nose- usually from overly aggressive septoplasty, an underlying autoimmune disease, cocaine use, badly healed nasal trauma, etc. Based on what you said about your breathing, it seems like you might have a septal perforation and/or loss of much of your septum but it is hard to know for sure without looking inside. The long term solution is you need to see a revision rhinoplasty surgeon to discuss reconstruction.

Good morning. There can be many reasons for saddle nose, but yes, saddle nose can cause breathing issues. Fillers may temporarily camouflage the depression, but more permanent, long term solution would be a revision rhinoplasty, both for function and cosmesis.