I know that protectionism and strong industrial practices were characteristic of Japan from after World War II on and that the repeal of tariffs corresponded to the slump in the Japanese economy in the “Lost Decade,” but what exactly marked such a strong shift in the stability of Japan’s economy?
The problem isn't so much that it "crashed so quickly" -- its that it crashed so _slowly_. Japan had a notably different approach to financial markets from the the Anglo-American model. Yes, there was a stock market, but the public float was less important than industrial group cross holdings and visible price discovery for equity and debt was very slow; banks kept zombie borrowers alive for years, leaving little capital for more potentially useful borrowers.
The Japanese tended to finance their debts through bank loans, in webs of interconnected companies -- there were four big zaibatsu (Sumitomo, Mitsui, Mitsubishi, and Yasuda) and a bunch of smaller ones, and these obligations, cemented by reciprocal cross shareholdings, were a big part of how you got and stayed funded.
What this meant was that there were a ton of loans from banks which were not "arm's length" -- and when they couldn't get paid, banks didn't write them down or sell them off, they just kept loaning their affiliated companies more money to keep paying the debt.
This, together with the aging of the Japanese population, created a deflationary feedback of falling asset prices, growth in problem loans and falling investment and consumption.
Ben Bernanke identified the problem as one of banks awash in bad debt, unwilling to make loans, and consequent monetary stagnation, explicitly calling for a more dramatic approach instead of more "muddling through":
Japan is not in a Great Depression by any means, but its economy has operated below potential for nearly a decade. Nor is it by any means clear that recovery is imminent. Policy options exist that could greatly reduce these losses. Why isn’t more happening? To this outsider, at least, Japanese monetary policy seems paralyzed, with a paralysis that is largely self-induced. Most striking is the apparent unwillingness of the monetary authorities to experiment, to try anything that isn’t absolutely guaranteed to work. Perhaps it’s time for some Rooseveltian resolve in Japan.
If you contrast with Korea, which had a panic, a crash, and an international rescue in 1998, the Japanese experience -- in which "The Lost Decade" has stretched to almost 20 years, measured on the Nikkei-- seems to be a case of "trying to avoid a meltdown caused more problems than it solved"
What was the specific stimulus in 1991 that caused the decline? There wasn't one. It was the accumulation of excesses, and the decline could have started in 1990, or 1992; the length of the crisis is evidence that it wasn't linked to some particular event.
Sources:
Understanding the Costs of Deflation in the Japanese Context
Japanese Monetary Policy: A Case of Self-Induced Paralysis? -- this is a very nice paper by Ben Bernanke, written while he was still an Econ professor at Princeton, worth reading both as for its analysis of Japan, and as an indication of how he thought about deflationary spirals _before_ the US financial crisis of 2007-2009
Korea makes a useful comparison:
THE POLITICAL ECONOMY OF FINANCIAL CRISIS IN SOUTH KOREA: FROM ECONOMIC MIRACLE TO FINANCIAL CRISIS