S&P500 and EUR/USD

Hi all,
Could anyone tell me what affects how some currency pairs,e.g. EUR/USD vary with the direction of S&P500 in the NY session.Sometimes in the same direction and others in the exact opposite direction?
Any clues/hints is appreciated.
Cheers,
Angie

SP500 & EUR/USD are both denominated in USD, so they will correlate from that side and that's a certainty.

on the flipside, any pair that's quoted as USD/XXX will have inverse correlation to SP500 because of the switched USD.

their volatility may also correlate because they share the common NY session activity (liquidity & money flows), but that doesn't guarantee direction.

so for the independent currency directions.. i'd imagine they may be driven by sticky fundamentals and change gradually, or something else and change 'randomly'...

need to run some statistics, or someone can post theirs.