According to Wikileaks, the official reason is for criticizing the detainment of the Catalan separatist Carles Puigdemont.
The Wikileaks twitter account tweeted on March 28th:
Wikileaks editor @julianassange has been gagged and isolated by order of Ecuador's new president @Lenin Moreno. He cannot tweet, speak to the press, recieve visitors or make telephone calls. Ecuador demanded that he remove the following Tweet:
The tweet in question by the twitter account of Julian Assange from March 26th:
In 1940 the elected president of Catalonia, Lluís Companys, was captured by the Gestapo, at the request of Spain, delivered to them and executed. Today, German police have arrested the elected president of Catalonia, Carles Puigdemont, at the request of Spain, to be extradited.
However, on March 29th, the Wikileaks twitter account later claimed:
Although Ecuador claims it isolated Assange over his Tweeting about the detention of #Piugdmont in Germany, the political context is his breaking the "Watergate" of Ecudor, #HackingTeam, which led to the implosion of the national spy service this month
Whether or not Wikileaks is a trustworthy source in this matter is up for the reader to decide.