This may not happen all the time. It happened with on of my friend that as soon as she started frying vadas suddenly the vadas from hot oil started rusting and popping. She had to rush out of kitchen, after cautiously switching off the gas. Her mom and my om stated the same reason- aeration of batter. This might have happened as the batter was not very well aerated. She didn't beat the batter till it was airy, and add lot of vadas in hot oil. Lack of aeration and space in kadhai (as vadas expand on frying) might be one of the reason. So, be careful while frying the vadas and aerate the batter properly.