The reason the market may go down is because it is over valued based on the support of the fed through artificially low interest rates and other measures. Trump will fire Yellen creating instability and the market will adjust.
Trump cannot fire Yellen. That job is a six year appointment not subject to firing, non-political you know. Nor can he fire the Director of the FBI for the same reason.
All that can be done is not re-appointing them and I think Yellen was just reappointed a year ago or so.
But to me thinks like keeping interest rates low, forking out $85 billion/month for T-Bills and Mortgage Backed Securities (for over 6 years, IIRC) artificially propped up the market in part. But what that did was increase our National Debt by 1 Trillion dollars - that money spent on Obama giveaways and favors.
Trump stopping that isn’t tanking the market; it is bringing it back up to a level playing field in my book.