My theory is that over long months of negotiating, Assange has been assured and convinced of his eventual freedom by President Trump which will be granted to him after he testifies.
Also, I think POTUS in his meetings with Teresa May, guaranteed solidified a deal that Assange would be extradited to the United States.
I think Ecuadorian Embassy is releasing Assange on Assanges's permission and POTUS's request. Assange understands that by testifying he not only gains his freedom but also
he wants those responsible for Seth Riches murder to come to justice.
Seth Rich murder exposure would open the gates to a plethora of crimes and set the justice ball rolling toward Hillary/Obama and company
Of course any charges leveled against Assange would be pardoned by POTUS
Assange's extradition could be the catalyst we have all been waiting for.