Auto body repair & detailing: auto body repair, auto body repair, lower quarter

Question
Thank you in advance for your help!
I Bought a certified used car , a few weeks later I noticed an "X" shaped crack in the rear lower quarter panel.  I put a magnet on it, and there was body filler there.  The dealer repaired it for free in their body shop.  Six months later, same crack has appeared again.  Any idea why?

Answer
They didn't properly repair the filler cracks. The old filler needs removed entirely, and new filler applied. When filler cracks, it also cracks the topcoat letting water intrude into the repair. Once that happens, rust starts setting in. They probably just cut a "V" into the cracks themselves, then filled over them without removing the old filler entirely. That is why the cracks are back in the exact same spots. Bill