I agree with MBSims. Transverse weld metal cracking is usually hydrogen cracking of the weld metal. Because weld metal has a lower lower carbon equivalent than base metal, it usually only occurs in high strength weld metal. I have only seen it on 120ksi electrodes and higher, but it can happen on lower as well. As MBSims mentioned, the strength (hardness) of the weld metal and the cooling rate both affect the cracking. While the 100F preheat might have been high enough to meet the recommendations for the base metal, the weld metal in this case requires a higher preheat. Make sure there are no contaminants contributing any hydrogen as well, such as rust, poor shielding gas, cutting fluids, etc.