Could be a thousand different things that caused this. Prep? Did you clean, scrape, sand, clean again before applying primer? Was there excess moisture in/on the surface when you painted? Did you use quality paint? Is the garage heated? Insulated? This could be a factor in the peeling of the exterior. Did you have a hail storm? Hail does horrific damage to ANY painted surface. Could the gutters on that side of the garage be plugged up allowing water to run over and onto the painted surface? Really could be any of those things or none of those things.