Firstly, a player receives 100 points in the Weekly Siege Ranking each time they besiege a Castle.
The Weekly Ranking is updated every Monday at 7:00 AM UTC for all players worldwide. At that time, all Siege points earned last week reset to 0. Hence, when a player starts recalling troops from the besieged Castles, each time they recall their Units from a single Castle, they get negative 100 Siege points.
Suppose a player notices that they haven't been in the Siege Rankings for several weeks, despite having laid a lot of Sieges. This means that at one point in time, they must have recalled their Units from too many besieged Castles after the Monday reset. This, unfortunately, leads to a negative balance of Siege Points.
There are two solutions to this issue:
1. A player may need to besiege new Castles and earn Siege points to reach 0. After that, all further Sieges will bring them Siege points for the current Weekly Ranking.
2. The easier way is to wait until next Monday, when the current negative value will be reset to 0 automatically. However, it is important to note that a player needs to recall their Units from all besieged Castles.