A Psychology Today article points out that a many vegetarians eventually go back to eating meat. The most common reason for the switch is that they just didn't thrive on the veggie diet.
As an ex-vegetarian, I can say that this was my exact reason. I feel much stronger and healthier now. Also, I happened to switch back to meat at the end of nutrition school. Once I learned how the human body works, I realized that quality protein is essential to manufacturing brain chemicals, and for growth and repair. Now, some vegetarians will argue that they do get protein, maybe, but often it's from processed foods such as fake meats and soy. Even if they are a whole foods vegetarian, some people simply cannot thrive on this diet.
In the survey the average time for going back to meat eating was nine years. This makes sense as our B12 stores diminish without animal protein. Contrary to what many veggies believe, the plant sources of B12 are not the same as what the human body needs.
Largely, ex-vegetarians who become omnivores choose quality and ethically raised animals. This is also true in my case. My nutrition teachers had been veggie for over 20 years, and they mentioned that in the 70's there jut weren't local and grass-fed sources of meat available. Many people switched back once better choices became available.