Icing is caused by supercooled raindrops freezing on the airframe. In this case the spacecraft came from a very high altitude so I'd expect the airframe to already be very cold and even if droplets aren't supercooled, they might just freeze on the cold surface of the lens. Pireps (pilot reports) are available globally where pilots report icing conditions they encounter. I suspect the SpaceX engineers anticipated this possibility and decided that they didn't want to add the extra weight and complexity of a heating or deicing system to the lens.

I have a feeling someone is working there right now to come up with a quick and dirty deicing system - considering how disappointing this video is. The fluid system the Cirrus SR22 uses comes to mind.