1) The cones might have been roughly handled, resulting in fine hairline cracks. These can break during firing.
2) If the Kiln Sitter sensing rod is binding at the pivot point, it can exert too much pressure against the cone. Binding of the rod can be due to glaze buildup inside the porcelain tube. Make sure the sensing rod can pivot freely.
3) A very fast firing can break the Kiln Sitter cone.
4) The Kiln Sitter could be out of adjustment. Please adjust every month or so with the Kiln Sitter firing gauge.
The cone has a number stamped onto one of the sides. You should place the cone in the Kiln Sitter with the number side down. That side is the smoothest of the three sides. Placing that side down lessens the chance of breakage.