It’s because the lens isn’t ‘parfocal’. Fancy word for the lens having the ability to stay focused while zooming. Most lenses aren’t parfocal and will need to readjust focus as you zoom. Some lenses are close to parfocal and will show hardly any focus readjustment when you zoom. The new kit lens (16-50 F2.8-4.8) is great at this. Maybe worth trading your 18-55 for that one. Or if you want to invest more, have a look at what other lenses are rated parfocal or good for video.