Switching between 3G/WiFi has been a known source of libspotify problems in the past. It's possible that there is a crash with libspotify itself, I've sent a mail to the internal group which maintains libspotify to ping them for more information (I work at Spotify myself, but not in the libspotify team).
If the crash is coming from libspotify, there's not much you can do to guard against it, the best way is to abstract libspotify in an Android service (which I presume you are already doing) and then restart the service from the UI APK if you notice that it crashes. This might cost you a few seconds of waiting but it's better than the entire app going down in flames.
Edit: There's a new Spotify SDK for Android, which replaces libspotify and is much easier to develop with. You should migrate any libspotify projects to the new SDK.