Same thing with my Belkin iP7+ screen protector last year. Applied in the Apple Store and within a week a small crack had appeared (thin line) on the top right of the phone. Honestly: It never fell, no enormous amount of pressure was applied... really bad quality... Took it into the Apple Store: They didn't care and told me it must have "done its job". Called Belkin: They didn't care. It's easy to just rid complaints down to the obvious "you must have dropped it" and the customer has no way of proving the opposite.
Recently bought 2x Anker Glass Protectors (€7,99 on Amazon) and they're holding up well. Application was easy with the included kit and the protector is a little wider than the Belkin one which means the edges of the screen don't get this "rainbow" effect. Cheaper, better, ... Belkin? Never again.