Reuters - Last Thursday morning investors queued around the block to buy the bonds of recent defaulter Greece, and by the end of the day were selling U.S. tech stocks furiously.

In an investment world so used to the concept of 'risk on' or 'risk off' over recent turbulent years, the behaviour was puzzling. So what happened?

Observers point to a combination of three drivers - stocks valued too highly, global growth failing to meet expectations and underlying investor behaviour since the turn of the year - but curiously no single catalyst.

Many stock markets around the world, including those in both developed and emerging countries, are at or near their highest levels ever thanks to central banks propping up the global economic recovery with their ultra-loose monetary policy.

In the United States the S&P 500 has almost tripled its level since the post-crash trough in March 2009, while the Nasdaq Composite index weighted more towards technology stocks has gained almost 250 percent in that time.

This meant that at the end of March, U.S. stocks were the third most expensive in the world behind Japan and Mexico, analysts at Barclays estimate.

In its latest monthly poll of global fund managers, Bank of America-Merrill Lynch said the proportion of investors who think stocks are "over-valued" is now the highest since July 2000, just when the Nasdaq was in the early stages of collapse.

So perhaps a correction was on the cards.

Assuming that to be true, the readjustment was most likely to be seen in stocks most sensitive to the second driver, a reappraisal of global growth expectations.

And that's exactly what happened. Last Thursday's 3 percent slide in the Nasdaq was its biggest since November 2011.

Two days before that, the International Monetary Fund trimmed its 2014 world growth forecast to 3.6 percent, the fifth downward revision from its last six projections, and OPEC lowered its 2014 forecast for oil demand, traditionally a closely-watched barometer of global economic activity.

"I'm not sure what the trigger was, but the market moves do suggest that investors are lowering their sights on growth and expect rates to stay low for longer. Hence equities down and bonds up," said Joachim Fels, chief international economist with Morgan Stanley.

ROTATE TO REFLATE

The asset class that benefits most in this environment is fixed income, especially euro zone bonds if the European Central Bank pulls out all the stops in its fight against deflation, potentially keeping interest rates super-low for years to come.

Demand for Greece's five-year bond, which marked Athens' return to the bond market only two years after defaulting, topped 20 billion euros ($28 billion). More than 500 investors bid for them.

Was that a staggering display of investor confidence, or symptomatic of something more deep-rooted? According to research by JP Morgan's Nikolaos Panigirtzoglou, it's the latter, the third of the three drivers.

If the principal investment theme of 2013 was the "Great Rotation" out of fixed income into stocks, the early indications this year point to a partial reversal of that flow to a broader "Asset Reflation".

Last year the world's major stock markets rose between 20 and 30 percent, while 10-year U.S. Treasury bonds, the global fixed income benchmark, lost almost 8 percent on a total returns basis.

Investors are now no longer selling bonds to buy stocks but are instead buying both stocks and bonds equally. The net effect is stronger bonds, and on a relative basis, weaker stocks.

The pace of equity fund buying by retail investors slowed to around $75 billion in the first quarter, 50 percent of last year's pace, JP Morgan's Panigirtzoglou estimates.

And bonds of all stripes - even Greek ones - are in demand.

"The implications of this shift are twofold: there is no retail flow support for the bearish view on bond markets, and there is significantly less retail money flowing into equities than last year," Panigirtzoglou said.

It's also worth bearing in mind the relative size of the global equity universe.

At the end of last year the stock of global financial assets stood at $242 trillion, according to Deutsche Bank. Of that, equities accounted for only $64 trillion and fixed income assets were worth some $97 trillion.

Global stock market capitalisation was worth 87 percent of all goods and services produced in the world economy last year. The value of outstanding bonds was worth 133 percent.

In short, it doesn't take a huge relative shift to magnify the impact on stocks. The flip side to that is it takes a sizeable flow in or out of fixed income to really be felt.

So why the shift since the beginning of the year? Again, there's no clear answer. JP Morgan's Panigirtzoglou says investors may have simply realised the pace of last year's stock market rally was unsustainable and so decided to buy more bonds.

Alternatively, they were attracted by the relatively high yields offered by bonds and began to snap them up, just as pension funds and insurance companies did late last year.