There ya go!
When the CPU is running, it trips a counter to keep the watchdog at bay. If it's not running then the watchdog itself will trip and smack the reset line in an attempt to get the CPU running again.
BUT...
If you have a problem with the CPU, program ROMs, program RAM, address decoding, address or data buffers, interrupts, and more, you'll end up with a watchdog timer tripping the reset line. Gouged traces in those areas of the board will do it too, as will corroded traces from liquid or rodent damage.
If the reset line had never done this before replacing something then check what you replaced. Did a pad or platethru get pulled? Did a pad or trace get bridged with solder? Was the replacement installed backwards? Was the replacement ROM programmed with the correct code?
If the board does it right off the bat then check for gouged traces and the items listed above.
Have fun with the ROTJ board! There's a HUGE satisfaction that comes from fixing a board.