Honestly, he's been in office around a year now. This is the transition period. Maybe this is some kind of correction that resulted from something Obama did? Who knows?
The problem of course is that Trump has been taking credit for the gains in the stock market for a long time before this. So either this dip in the stock market is his fault, or else he was lying all those other times before this when he claimed credit for the performance of the stock market.
Maybe this stock market decline is Trump's fault, and maybe it isn't. The important thing is that Trump supporters have put themselves in a box. If they insist that Trump was telling the truth all those times he took credit for the performance of the stock market, then they now have to blame Trump for this decline in the stock market. If they insist that this current decline in the stock market is not Trump's fault, then they just admitted that they and Trump were lying all those previous times they attributed rises in the stock market to Trump.
So no matter what answer conservolibertarians give, they reveal themselves to be full of shit, which is why they will probably use
special pleading fallacies to insist that the previous gains in the stock market are to be credited to Trump, but the current decline is to be attributed to Obama.