This is caused by a change the grading table while the teacher attempts to grade a student in the single grading page. Moodle uses a rownum value to determine which student the teacher is focusing on.
The rownum can be seen in the example URL below.
http://examplemoodleurl.com/mod/assign/view.php?id=95&rownum=0&action=grade
Row number of 0 means the first student (record at the top of the page), rownum 1 would be the second and see forth. A change to the grading table can be cause by anyone of the follow.
1. A student enrolled into the course
2. A student unenrolled from the course
3. A student saved a draft assignment
4. A student has submitted an assignment
5. A student has been moved from one group to a next
6. The user has multiple browser tabs open and has clicked on a table header to sort the table.
If the grading table was to have a change and the teacher refreshes the single grading table, the student being focused on might change. If the teacher was to submit a grade and/or feedback after the grading table has changes, the data would be assigned to the wrong student.

See word doc attachment for more details.