1 Ran out of money is often the explanation.

Note that Asonomics is the usual statist recipy: inflate government debts away, pile up taxes sky-high, use the proceeds to buy votes and create a massive dependent class. Even the American inflation cannot keep up with the debasement of Yen by Aso's government.

2 The Bank of Japan can't run out of Yen, just like the Fed can't run out of dollars. They create as many as they need out of thin air.

3 Technically, while sovereign banks can't run out of their own currency, they CAN make so much of it that people who have any choice stop accepting it. And, eventually, everybody has a choice.
Or, to put it another way, anybody who buys groceries knows that American inflation is a lot higher than the government admits. For Japan to keep their currency at parity with the US dollar would be to replicate American inflation. Maybe they've decided they don't want to go there.

4

Actually, if the yen is rising relative to the dollar, it means Japanese inflation is higher than ours.

Which, admittedly, may be why they stopped.

5 This seems to be full of relevant information on what's happening to the Yen.

-j

6 Note: I keep up with what is happening with currencies as part of work.
While in a "normal" market, I'd agree with your analysis, but currencies aren't a normal market.  As a result, it's actually the reverse.  The exchange range hit the range between 103 & 101.20 and stayed there for 4 months because the Big Players were *out* of the market.
Currency markets are controlled by the Central Banks of their respective countries.  However, pretty much the USA, UK & Eurozone are the only Central Banks with complete control over their exchange rates.  The BoJ can be pushed around by the big banks & sovereign funds.
What has really been going on all year is a rolling set of Forex Collusion investigations at the major exchanges.  This put a lot of the major players on the sidelines, until they worked out how to operate within the new rules,  plus a lot of what is going on within the major economies hadn't been sorted out.
However, the current run has a lot to do with the rapid devaluation of the Euro.  The value of the USD is heavily related to that exchange rate, and as the Euro dropped 600 basis points, it allowed them to move the Yen higher.   There's also functional macro-economic reasons for exchange rates to move during certain parts of the year.  This year's Fall move just happened to be bigger (mostly as the majority of the year had been much tighter).
We're approaching an area for a pull back, as they've got orders to pull in & stack up before trying to move higher.  And the Fed's QE3 ends very soon, which should cause an equities sell-off. 
So, now's a good time to buy Japanese goods, if you want some.

7 As for the Japanese economy, all of the talk will always miss the point: the problems are baked in and unsolvable.  Japan will functionally default at some point.   It's just a matter of when.  The bad debt is piled beyond what is ever capable for the country to pay off.
However, all of that Debt is held by Japanese Banks & the Bank of Japan.  So, it's both eating the country apart & it doesn't truly exist.  How's that for a problem that can't be unwound?
Since Japan is prelude to what Europe is going to go through, and the USA will be looking at after the equities explode again, it's because of a "debt hangover" in the real estate markets.  Politicians can't allow the time for the market to re-balance itself.  So they add more debt to solve a debt problem.  No structural reform will change the calculus.  There isn't any way to solve the problem *now*.  (200% of GDP ago, there was)