XJO Weekend Charting and Chat - 09 Feb 2018, page-24

@redbacka has shown many times that the XJO and S&P are only loosely correlated at best.
The actual numbers vary at different times during the year.
That said, when the US (which has the biggest and most powerful markets around the world), strikes some difficulties, most markets around the world feel some of the damage as well.
There are various reasons for this, but for me, the most obvious is because a big event overseas will always cause to selling pressure here, and no one wants to stand in front of the difficulties and attempt to 'hold it up' single handedly, not even professional traders (they are often more than happy for it to go down for a while, with the offer of some nice cheap stock in the future).

Market trading almost always takes the path of least resistance, and if selling pressure is strongest at a certain point in time, then the easiest path is lower, and that is where price goes.

That said, once the serious supply begins to reduce, at some point prices appear cheap enough to attract buying, and the market begins to turn.

cheers