Switching between QCAR- and other Activities

Hello, we use ImageTargets as a base for a simple test application. Depending on the detected marker, the app should either display a 3D object or play a video file via the MediaPlayer class. This works pretty good, but as soon as we finish to the MediaPlayer activity (by pressing the back button), the app re-initializes (showing the splash screen and reloading all our OpenGL stuff). We want to prevent this, because it takes quite a while (10 to 15 seconds) since the app basically starts all over and reprocesses our 3D models. But we can't really figure out why this happens. We first thought that the MediaPlayer destroys our OpenGL context, but this doesn't seem to be the case, since even if we just start a very basic activity that displays a Toast, the app reloads. If we send the app in the background and reopen it, it doesn't need to reload and is instantly in tracking mode again. Any hints on what could be wrong? Thanks.