Mechanism has never been worked out. But it always augurs well for the graft in my experience. In those instances where AVF has closed without hypotension , the graft has had excellent function

Agree with you K.Sampathkumar sir, never had a satisfactory answer for the same from my seniors too. Is post transplant correction of uremic platelet dysfunction and at times erythrocytosis contributory to spontaneous closure? 

Plausible explanations. 

I believe it is at related not only to improved platelet function with the end of uremia, but also to the loss of assisted primary patency (i.e. we are no longer keeping these grafts open actively).

Improvement in coagulation system

In my limited experience , several factors seems to be contributing. As it is low flow system , lack of hand exercise and attention to keep patent , and in absence of regular flow as a result of cessation of HD following successful transplant might play a role. Improved coagulation with improvement of uremia , increases viscosity with erythrocytosis may lead to thrombosis of AV fistula and spontaneous closure. 

In my experience, it is not related to physio logical improvements

We have created thousands of arteriovenous fistulas (AVF) for hemodialysis (1978ff.). The persistent uremic metabolic condition with lower hemoglobin levels seemed to be protective against thrombosis and pulmonary embolism in hemodialysis patients. The reversal of this uremic condition after renal transplantation and thrombosis tendency postoperatively favor thrombosis and closure of AVF.