Sri

There is usually some water content in the crystals of KCl if left exposed to the atmosphere. When heated rapidly, water vapour escapes suddenly cracking the crystals(with a sound, of course). I wouldn't expect it from anhydrous sample. KCl melts at 770 C and alumina at above 2000 C. You might not have reached the temperature needed to melt alumina. regarding SnO2 it could possibly be due to similar processes seen in ZnO(see wiki)