There are multiple factors which have contributed to the rise of "vegetarianism" food habits. Although I disagree it has risen only in past few years, it has been growing consistently over several decades. Let's look at these factors closely
Factor 1: Migration from Indian subcontinent
Vegetarianism as a stable food habit existed mainly in Indian subcontinent (India and its neighbours). There is a massive migration wave from India for last 100 years or so. These have established vegetarianism food markets, spread cooking styles and dishes in almost every country in world, specially USA.
Importantly, recent rise of Indians at world stage whether its science, economics and politics has added to the spread of "Indian" way of doing things.
Factor 2: Health
Rising health problems is severe concern among high income groups. Although they spend a lot on good food, health problems are not going down. Meat products were severely marketed over plant products before as major source of protein, important for sound growth. However, such myths have been busted. Health practitioners are increasing advocating reduction of red meat consumption to enhance health.
Factor 3: Business of Vegetarianism
Although plant products are cheaper to produce than animal products by several factors, their costs in market are at par with each other. How? There is lot of branding around "organic" going on and they are pushing their products over meat based.
Factor 4: Spread of Yoga
Yoga has been accepted increasingly among world populations, specially higher income groups. Yoga has proven to be productive and has been endorsed by people in variety of fields. There are many among yoga practitioners who are adopting "vegetarianism" form of lifestyle.
Factor 5: Spread of Hinduism
Since Vivekananda, there has been growing interest in Hinduism across the world. Currently, several gurus have established centers for spreading Hindu philosophy, including their eating habits and associated whys and whats. Very briefly, food items are divided among "Sattvic, Rajsic and Tamsic" which loosely translates into "Pure", "Productive" and "Destructive". Several plants, like onion and garlic, fall under "Destructive" category (and hence are prohibited from consumption among many stricter vegetarians!). Similarly, some animal products fall under "Pure" category, like Milk and Honey. Nevertheless, it does loosely generalise into "plants products are purer than animal products".
Factor 6 : Global warming, Animal Cruelty, Internet etc
Raising animals for food contributes significantly to green house gases. Concern for global warming among people have risen. They are adopting lifestyles having less carbon footprint.
Animal cruelty is a very personal subject, however, large economies have taken strong steps to reduce public consumption of animal cruelty (like films).
The internet has spread practically everything good and bad, whether its terrorism or vegetarianism. So I'd call it a neutral factor.
