There is a significant change in the Windows Phone 8.1 webview and apps, compared to the "legacy" Windows Phone 8.0 webview and apps that is the cause of many Windows app crash problems.
A "legacy" windows phone app is easily recognized as a XAP file.
A "universal" or "modern" Windows app for Windows Phone has an APPX extension.
At this time, the XDK Windows Phone 8.1 build tile will build an APPX file using the Cordova "windows" target.
Legacy XAP apps are built using the Cordova "wp8" target, which is considered obsolete by the Microsoft Cordova project team, and is not currently supported by the XDK build system.