Your phone does a few things automatically to reduce mobile data costs. If there is a known WLAN connection available, your phone switches to it immediately. To minimise data costs, and also to save battery, Maps enters stand-by mode when you swipe to home.
You can change the settings to disable background internet connection to further save on data costs. However, for the best user experience, it is recommended that you contact your operator to invest in a fixed fee data plan that prevents unexpected data costs but provides freedom to experience your phone as it’s intended to be.