Why does BOJ unchanged rate lead to Nikkei and Topix declines? Why does the BOJ's decision to leave rates unchanged lead to a stronger yen and declines in the Nikkei and Topix? Thanks in advance!
It doesn't in general.

It did in this specific case because the Nikkei and Topix had earlier priced in the possibility that the Bank of Japan would do something other than leaving rates unchanged this time. Now that that possibility has gone, the market no longer needs to price in the possibility, and so has corrected.

That's not just specific to this case: that's true of all market responses when an unknown event becomes known.