Gangkye Drubpa Kyab is taken into custody after attending a reception welcoming his early release from prison.
He is freed without explanation after serving more than four years for criticizing Beijing's rule in Tibetan areas.
Usually held only by monks, the event draws hundreds of nuns from seven convents in Sichuan and Qinghai.
A resolution of the dispute over Tibet's status has been hindered by separate security concerns, a Tibetan scholar says.
Both monks and laypeople take part in religious instruction and promotion of Tibetan language and culture.
He had watched a program in the prison's TV room outside permitted hours, a local source says.
He joins another Socktsang monk in custody after being seized at gunpoint in his quarters.
Sentenced for having supported a self-immolation protest, he is freed for 'good conduct' while in custody.
Described as involved only in the monastery's work, he may have been targeted for viewing news broadcast from outside areas.
He is warmly greeted at home by relatives and friends despite police warnings that no welcome should be given.
Lobsang Kelsang's family still lack information on his condition, though, and are not allowed to visit him.
Two more nuns have died, with a third saved at the last minute by friends who intervened.
Houses, cars, and livestock are swept away, but no deaths are reported.
Authorities approve festival but deploy police, fearing possible protests against Chinese rule.
Jailed before for calling for Tibetan independence, he was imprisoned again for giving information to 'outside separatists.'
Chinese authorities meanwhile ban religious ceremonies at Larung Gar for fear they may draw large crowds.
The order so far affects only residents coming from the neighboring Tibet Autonomous Region, sources say.