What is exact reason for "Unfortunately, app has stopped"? This error seldom occur for system apps but sometimes it occur while open an application that newly installed and that app doesn't start.

Question isn't about specific app or process but exact reason and possibilities of this error occurrence.
From a software standpoint, there are many types of errors.

All errors should have the ability to be handled as to let the application remain running, but sometimes there are errors that are uncaught. The developer is responsible for catching the errors or handling things to not allow errors to occur. However, there are some things that a developer is not able to foresee, so they slip past.

These uncaught errors can crash the application. They can range from any number of things, such as database access, file reading, values not being properly checked, etc. The program ends up trying to do something it is not capable of doing, and simply crashes.