The most common causes:
The trigger is out of adjustment
The porcelain tube is corroded.
The actuating rod is bent.
An obstruction outside the kiln is preventing the weight from dropping. For instance, leaning an object against the kiln or placing a chair too close could prevent the weight from dropping.
Ceramic ware inside the kiln falling against the actuating rod can cause an over-fire.
A cone sticking to the cone holder has caused an over-fire. (This is why you should coat the cone supports and actuating rod with kiln wash.)
Periodically examine the cone supports (the two metal prongs that hold the cone in the Kiln Sitter.) If pieces of cone have stuck to the supports, slide the supports out of the protection tube and grind off the debris.
Always monitor the kiln, even though you have an automatic shut-off. The Kiln Sitter, or any other shut-off device, can fail. Nothing can replace the attention of the operator. Automatic shut-off devices are designed for convenience, relieving you from having to closely watch the kiln. But no shut-off was ever meant to eliminate the monitoring of the kiln.