SPY-TLT Relationship a Cautionary Tale

In reaction to today's strong ISM data, the SPDR S&P 500 (SPY) has rocketed towards a retest of its April 2 high at 142.21. Meanwhile, the iShares Barclays 20+ Year Treasury Bond (TLT) has weakened, but curiously it has not imploded in a way that would suggest either that the economy is stronger than most people think or to allay suspicions about sustainable strength in the equity indices.

If one chooses to view the SPY-TLT relationship as a cautionary tale, then the former is topping, while the latter is pulling back ahead of another up-leg.

Conversely, if one is to remain extremely bullish about the S&P 500 Index, then how to reconcile the stubborn and constant "bid" at the long end of the bond market?

Is this all about the Fed underpinning the long end of the bond market, or could something else be going on as well?