It's actually quite the logical rule. In a mutual-destruction situation, the Hero has the advantage and first initiative. So he would pass, giving the defender a chance to play something to get out of mutual destruction. If he did not, the Hero then has the final option to terminate the battle or play something to get out of mutual destruction.
This doesn't happen in big tournaments because SA's are more important than numbers, but when newbies are playing with less SA's and more number boosters, it's important to know exactly how initiative passing works.