This isn't the first time Assange has been cut off from the internet, though the last time his Twitter account went silent may have been a fluke. Before that, the Ecuadorian government also temporarily shut down Assange's access to the internet after WikiLeaks published information from Hillary Clinton's campaign.
As translated by a Twitter user as a reply to the original tweet, the Ecuadorian government says that it suspended "the systems that allow Julian Assange to communicate with the outside world" from within the embassy itself. Apparently, Assange breached a written agreement to not send messages that interfered with other governments. While not naming Assange's last tweet, which fired back at a UK minister who called Assange a "miserable little worm," the document asserts that Assange's tweet "puts at risk the good relations that the country maintains with the United Kingdom."