iOS 7 zooms in and out when between applications, causing dizziness and nausea for some users, similar to the effect of seeing stable objects while on a rocking boat, thus confusing the brain into motion sickness.
If you’re experiencing this iOS 7 side effect, try going to Settings, General, Accessibility, and turn on the Reduce Motion setting. This should help to relieve that motion sickness sensation.
For more information- http://healthland.time.com/2013/09/30/ios7-making-you-dizzy-heres-why-and-a-fix/
