When he is losing, sometimes he sorts of "give up". He starts going for big shots, aggressive tennis and his opponents most of the time get scared. They get used to reserved, safe, patient Djokovic, and all of sudden when they are about to beat him, balls are coming back more often and with more pace. Fear kicks and boom they lose.
His clutchness is actually a trick, being able to completely give up in a match and at the same time still want to win it. He is not playing under extreme pressure in those moments, he is playing under no pressure at all.