Looking at a phone screen, or any type of screen that attempts to emulate physical motion, can confuse a user's senses and induce motion sickness. Similar to the effect of 3-D movies or reading in a moving car, misalignments between a user's perceptions and the actual motion can cause sickness.
Apple's iOS 7 update may have caused motion sickness in certain individuals due to the software's emulation of movement within the screen when a user moved his phone. The phone software graphically moved around icons when the phone physically moved around, creating an illusion of motion. This movement, while visually alluring to some people, is physically disorienting to people's senses. Human senses interpret the icons moving around as their entire bodies moving around. This disconnect between sensory experience and physical motion is the crux of motion sickness, and it is why some smartphones can lead a user to become nauseated.
Some segments of the population are also more susceptible to motion sickness and are thus more likely to fall prey to any sort of graphical elements that create a rift between their perceived motions and actual motions. Many phone companies have recognized this issue, and they offer options on their phones to reduce simulated motion.