Glazes run for a variety of reasons.
Here are some of the reasons that glazes run:
1) It could be the glazes were fired to hot.
2) It could be that the glaze firing was too long.
3) It could be a mixture of glazes on the pieces ie. lowering the melting
point of both glazes
4) It could be a contaminated batch of glaze(s)
5) If the kiln is a computerized kiln the calibration of the pyrometer
could be off.