The reason why ipa crashes when modifying the identifier is a certificate issue, a code issue, or an Infoplist file issue.

1. Certificate problem: If your developer certificate does not match or has expired, the application may not be installed normally or crash. You can check whether the developer certificate is valid and update or regenerate the certificate.

2. Code problem: If you modify the application code during the process of modifying the identifier, and there are errors or conflicts in the code, it may also cause the application to crash. You can examine the application's code and try to resolve the issue.

3. Info.plist file problem: The application’s Info.plist file contains some configuration information of the application. If an error or conflict occurs in the Info.plist file during the process of modifying the identifier, , may also cause the application to crash. You can check the contents of the Info.plist file and try to resolve the issue.