According to the wsj article Trump began his hostile takeover of the Taj Maja in 1987 and used his stock holdings to sabotage the company so that it’s stock price would plummet and he could take it over.
The WSJ article says the stock tanked because RI was rumored as not able to obtain additional debt funding on their own. Up to that point, Trump, along with Griffin, had been buying boatloads of stock, so both of them would have taken any price drop in the pants right along with the rest of the stockholders. Even after the stock dropped, Trump offered to buy more but the takeover of Resorts Intl wasn’t a one-man show. Trump and Griffin didn’t begin to ink their settlement agreement until mid 1988 and Merv eventually walked away with Resorts Atlantic City and Paradise Island (the future Atlantis). And while Griffin was a household name which would boost shareholder confidence, with RI already on life-support there’s no assurances that RI would have remained whole under Griffin’s guidance or that the Taj would ever have been completed. (Merv later sold Paradise Island for about a quarter what he paid for it, if that says anything.)
Other things could have affected RI stock prices, such as the 1985 scandal involving a rather large bribe paid by RI to the Prime Minister of the Bahamas (see mention in NYT obit, below), the death of the company’s Chairman of the Board in 1986 with subsequent internal plays for control adding to the general uncertainty, or even the activation of shark repellants such poison pill clauses in the company charter.