I can't talk to vegetarianism in general, but the taboo on beef consumption is actually quite recent, developing only in the early first millennium--recent, I suppose I should note, in comparison to the way much of Indian culture is often portrayed as "timeless". In the practices of Vedic Brahmanism, which is the ritual set encoded in the Vedas and dominant in the aryavarta region until the early first millennium BCE, the consumption of beef was actually an extremely important communal ritual. However, after that the primary mode of Brahmanic distinction, by which I mean the manner in which the Brahman caste set itself off as more pure than the rest of society, became based on abstention rather than consumption. Possibly, this was a response to the holy asceticism of Buddhism, Jainism, and other Sramanic sects, or conversely it was because the growing complication of the relationship between religious and political power meant that the traditional consumptive role of Brahmans was no longer acceptable, or at least not in the same way. Regardless, it is important to remember that, at least in the first millennium, these dietary restrictions were not spread throughout the entire population.

I can't move it past that, but hopefully this can give some indication of how complex and changing this issue was.

The Heretic's Feast: A History of Vegetarianism, by Colin Spencer has a very interesting chapter focusing on India. It's available on Google Books.