A local chinese radio personality/tea expert once said that it was the pesticides in chinese teas that caused this, no foam/bubbles were detected with indian, sri lankan teas, can any teadrinkers verify this?
The bubbles appear when air is trapped inside a liquid having high surface tension.
In our tea, there's tea oil with lower density (therefore it's always on the top of the brewed liquid) appears as we pour water into it.
The air trapped inside this oil layer creates bubbles.
The amount of this oil (and thus also the bubbles to appear) depends on the geographic condition, soil and fertilizing technique employed to raise the tea plant. Moreover theoretically some post-harvesting technique might concentrate this oil content in the tea. At higher brewing temperature (100°C for black tea), the amount of bubbles should decrease along with surface tension decrease.