Issue on re-loading the interface instead it assigns a task in task tab

Hi All

When the user clicks the button on UI, the system has to load the same UI after executing certain logic/functionality in a sub process at backend.
But system just closes the UI and a task is been assign to the user in task tab. On opening the task, it opens the same UI and works fine there after.
No errors / warnings showing in the process models.

Any thoughts / idea what could be the issue

Thanks In Advance