Actually, vegetarianism is the only optional monastic rule, and since it was not optional in
Devadatta's monastic discipline, it suggests that vegetarianism was the source of the conflict.
In the past much of the offensive emails I received from Buddhist monks called me a modern day
Devadatta, because they believed that jhana was the issue of the schism. They erroneously claimed that
Devadatta's achievement of jhana caused him to become arrogant. Since the 8th fold is defined throughout the suttas in terms of the attainment of jhana, then we can conclude that these monks are clearly following a deeply flawed form of Buddhism.
However, I suspect that the conflict was deeper than the issue of vegetarianism. There are 2 other possibilities that occurred to me.
1)
Devadatta was delusional, because claiming ownership of the Buddha sangha after only belonging to it for a year is surely insufficient time to become a master, although jhana attainment is certainly possible in that time.
2) Another possibility for the schism is in the
Samyutta Nikaya there are suttas that refer to mass suicides among Siddhartha Gautama's disciples. Whatever drove them to the suicides is likely to be the cause of the schism, if the occurrences are coincidental.