How long was the wick? Had the candle been anointed with any oils? If so, how much was used? Was the candle altered or added to in any way?
There are many factors which can cause a large flame on a candle. And as said before, heat can crack glass -- especially uneven heat. It's why, unless it is a certain type of glass, glass cookware must be heated slowly. But a flame very close to one part of a glass holder can cause that part to heat and expand, while the cooler part has not expanded so much. Since glass is brittle, it increases the risk of cracking.