When a QML app is put in the background, the log starts to fill with OpenGL errors, the exact error message differing between phones. I have duplicated this behaviour with the "Maroon" example app, as well as my own app. On Samsung Galaxy S4, these messages are repeated:
W/Adreno-EGLSUB(26533): <DequeueBuffer:585>: dequeue native buffer fail: No such device, buffer=0x793f991c, handle=0x786a6458
W/Adreno-EGL(26533): <qeglDrvAPI_eglSwapBuffers:3590>: EGL_BAD_SURFACE
W/Qt (26533): eglconvenience\qeglplatformcontext.cpp:319 (virtual void QEGLPlatformContext::swapBuffers(QPlatformSurface*)): QEGLPlatformContext::swapBuffers(): eglError: 12301, this: 0x79313cb8
eventually the memory is filled and the app dies in the background if it doesn't receive focus again within about 20 seconds.
W/Adreno-GSL(26533): <sharedmem_gpumem_alloc_id:1393>: sharedmem_gpumem_alloc: mmap failed errno 12 Out of memory
E/Adreno-GSL(26533): <ioctl_kgsl_sharedmem_alloc:1494>: ioctl_kgsl_sharedmem_alloc: FATAL ERROR : (null)
This behaviour seems to differ a bit between devices, I have also tested on an older LG P990 which logs errors but doesn't seem to run out of memory.