Outside of dietary restrictions through religious doctrine, do we have a rough idea of when abstaining from meat and/or meat products became something regular people chose to do personally? And when it became significantly popular?
The distinction between religious reasons and secular reasons blurs as you go back in time. A lot of debate among ancient scholars was around whether or not animals had souls. Back then, something having a soul was often just another way of saying something was sentient or had a consciousness.
Pythagoreas was quite possibly vegetarian for secular as well as religious reasons around 500 BCE, but this can't be known for sure.
It's possible that the first vegetarians-for-secular-reasons lived among the Stoics around 200-300 BCE Some Stoics believed that eating meat was a luxury that humans did not need to engage in.
Around the same time, Porphyry, http://en.wikipedia.org/wiki/Porphyry_(philosopher) was writing about and advocating vegetarianism. Some scholars believe him to be one of the first vegans. Below is a quote from one of his most famous works: On Abstinence from Animal Food.
"to deliver animals to be slaughtered and cooked, and thus be filled with murder, not for the sake of nutriment and satisfying the wants of nature, but making pleasure and gluttony the end of such conduct, is transcendently iniquitous and dire."