The league tables are constantly shifting and there really is no way to predict the future as it all depends on external factors out of your control, unfortunately...
I see it has dropped by 8 on the complete guide which is definitely pretty big... I think this year especially is based on how some universities have reacted to the pandemic, it might be that Sussex didn't do as much as students demanded and this has reflected in their rankings? Overall I don't believe this year's rankings are a good representation of the university's overall performance but I guess that could be up for debate.