Your game may crash due to several possible reasons. Try the steps below to troubleshoot.
1) Close all other applications.
2) Check and update your OS version.
3) Check your network connection status (Wi-Fi or mobile data).
4) Change the graphic settings on Defense Derby.
5) Go to Settings > Applications > Defense Derby > Storage > Clear data and cache on your device.
6) If your existing account is linked with a social network, attempt to delete and re-install the application.