When WDI is left unconnected, the watchdog timer is serviced within the watchdog timeout period by a low-high-low pulse from the counter chain. In your circuit design, you connect WDI to gnd trough a 100k resistor, leakage current will cause there is not low states any more, so the internal circuts cannot work correctly any more. You can try with a smaller pull down resistor, such as 10K.