This is due to chemical oxidation of the green tea catechins into
orange theaflavin - while the tea is sitting in your cup it reacts with
oxygen dissoved in the water - same reaction as would happen to make
black tea in a tea factory except that then it is catalysed by natural
tea enzymes. Some green teas do this more than others: I suspect that
using freshly drawn water also increases the rate of browning.

Nigel at Teacraft