Well someone had good intentions here but missed the mark.  The 68HC11E has a COP (Computer Operating Properly) system.  When this watchdog timer is properly armed any problems in the code, or for that matter anything that may cause the operation of the normal program flow to stop, will time out and jump to the specified address (in this case the top of the code) that will re-start the MPU.  For more details on how this COP timer operates you can check out the information contained in the 68HC11 E series manuals under the CONFIG register and the COP Reset sections.  You can see from the information about the CONFIG register that it is actually stored in internal EEPROM and transferred to read-only latches at power up. Once the MPU starts operating there is no way to reset this register without a system wide reset.  To change anything in this register you have to write to EEPROM and restart the MPU for the change to take affect.  Well it looks like this step was never performed on the MPU so the NOCOP bit is always set, permanently disabling the COP system from ever working.  All the code is in place to re-arm the system and it looks to have no other problems, it is just turned off.  I guess that says something about the code, it doesn't have any places to get stuck in a loop because if it did the motor would just stop dead.  It probably is a good idea to enable this function if changes are made to the code that you are not absolutely sure are going to work.
    To get the COP operating again you will have to clear bit 2 of the CONFIG register at $103F.  Being this is a write into EEPROM you should then change your code removing this bit clear.  A better way would be to test this bit and if it is set then clear it.  This would only write once to the CONFIG register for your MPU thus saving wear and tear on the EEPROM.