During the start-up process, the outer most section of many rotor bar designs can heat up as much as 300 degrees Celsius in less than 10 seconds. The inner most section of the rotor bar closest to the shaft may only heat up 100 degrees C. This creates a warping of the rotor bar and a significant thermal expansion equal to many tons of axial pressure. A bar with a high resistance connection will have less current flowing through it and will not expand at the same rate. This differential expansion and non symmetric heating results in severe stress and an eventual crack. Left unchecked it will produce the same environment for a stress fracture due to differential expansion on the bar in the next slot.
To read a case study on broken rotor bars go to http://www.pdma.com/pdfs/cs/CS0402R.pdf.