In one of my beds I have America's Cup, a nice white. It has the thickest flower stems of all my iris, but apparently something goes wrong when I heavily water that bed, and half the stems tipped over so far 2 of them snapped OFF. None of my other irises do this, although plenty of them send up the bloom stalks at angles. It's only the bloom stem, not the actual rhizome, that is tipping and breaking. The rhizome is still nice and solidly rooted in the ground.

Is this an over watering for this particular variety, or a stem weakness made worse by the weight of the water on the blooms, or something else entirely?