I predict that dosing sugar will cause some bacteria to become pathogenic.
I have explained this on the section of "Quorum Sensing" in my article.
When you provide a carbon source, the bacterial populations bloom suddenly and cause that decrease in nitrates. But once it blooms out of control, its quorum gene will naturally cause it to release toxins to kill off and lower their population.
The toxins produced have yet to be studied on reef tanks but if the method works well for you Min, then congratulations!!!