That happened because you're still learning to drive. When you're learning a new skill, everything you do (controlling the car, keeping track of other cars, pedestrians, bicyclists, traffic signs and rules, making sure the car stays at the correct part of the road etc.) is a very conscious effort and takes a lot of focusing. If something distracts you at this point, even small things can disturb the driving. Once you have enough driving experience, all that starts becoming automatic. Eventually you're able to drive safely without paying almost any attention to what you're doing. Once you reach this point, you're also less distracted by what else happens while you drive.