The reason is that he was tweeting again in favour of Catalan Republicans, more precisely against the current wave of political prisoners that Spain is taking, including probably the last Catalan elected president, now under custody in Germany.
This is confirmed by Assange himself in his last tweet "I have no right to twit about political prisoners."
The guy is now a hero to many catalans. He will be an honorary citizen of the forthcoming Catalan Republic, and there are already talks of erecting a statue of him in the center of Barcelona!