After watchdog for dual-config is enabled, the FPGA seems like not work properly every dozens of seconds. Why?

FPGA communicates with another FPGA with PHY chip. Normally(watchdog enable checkbox not checked), the PHY link led is always off when communication is good. While after I enble watchdog for dual-config(watchdog enable checkbox checked) and set watchdog value to any value, the link led will be on for about 2 seconds and return to be off. This will also be repeated after dozens of seconds. Why? How will I use the watchdog for dual-config? I didn't run remote update after power-up and the FPGA just run application firmware(just ethernet communication with another FPGA using PHY chip).

Attached file is my fpga code for the dual-config.