This will normally be because of the processes involved when a research student submits their draft thesis.
The Research & Enterprise Training Institute (RETI) on receipt of your draft thesis have to formally notify the research student finance team that the draft thesis has been submitted, giving the date it was submitted. It is only after this notification that your tuition fees can be paused.
If you have submitted your draft thesis and you receive an email asking for payment of tuition fees, please contact the research student finance team @ researchstudentfinance@gre.acuk and a member of team will look into this for you.