1. After a US election, the historically most common trend is for a rise in anticipation and also as loose monetary policy is a given prior to any election. This has tended to cause a hangover the following year.
2. The US markets have touch all time highs recently - despite not particularly stellar corporate returns or signs of massive growth potential. The Price/Earnings ratios are well above the long-run average of 12.
3. Trump, is, err, mad. Whilst some of his policies are going to be good for growth in the US, like investing in infrastructure and trading off environmental protection for economic expansion, they are also going to be implemented badly - after all, few of his advisory picks are getting put in place quickly.
4. The over-blown reaction to his immigration executive order has made some large US corporates very jumpy - confidence is a key thing in the markets and this already has taken a knock. Yes, some of them are lefty-hand wringing types, but still, they run some very important companies for the US economy. Look at Brexit, all the big companies against and the FTSE regularly takes a kicking when they come out with the woe-is-me-shtick.
5. Protectionism - this is a two way street, good for US jobs potentially, but at the expense of international companies also probably inflation and slower GDP growth. These latter issues will be priced in before the former. Reducing trade won't be good for US markets.
All in all, the Trump bounce does not feel that sustainable in 2017, even if over 4 years things do work out for the best.
And finally, the old phrase is worth a mention., when the US sneezes the UK catches a cold...