Why students need to click "Submit to Turnitin" link before the submitted paper can be uploaded to turnitin server?

It is because the teacher has set the value to “Review Upload before Submitting to Turnitin” in the “Auto Submit” field, resulting that the student’s submitted files were not automatically sent to Turnitin for checking.

Teachers can check the Turnitin assignment settings under "Options" tab in the Turnitin assignment page.
By default settings, student's submission will submit to turnitin server immediately after uploaded on Moodle.
Turnitin options tab

If “Review upload before submitting to Turnitin” is selected under "Auto Submit", student will need to click “Submit to Turnitin” link in order to submit the uploaded file to Turnitin server.
Submit to Turnitin

To avoid students missing the submission deadline because of this problem, we recommend teachers using the default setting when setting up a Turnitin assignment.