Why does the group change when the contents of the file change?

Question

linux

I set a group for the file that does not include the owner of the file, but after changing the data, the group changed to the owner.
Example:
after creating a file - -rwxr-x--- user user some_file
after changing a group - -rwxr-x--- user another_group some_file
after changing a file --rwxr-x--- user user some_file

Answer 1

Why does the group change when the contents of the file change?

Because the data change is done in a way that changes the group. The file may have been deleted and recreated. Running a data change process under strace might answer that question.
Simply opening a file and writing data to it does not change the group.

Answer 2

Show the rights to the folder. There probably is sgid