What makes you think Romney's a factor in this? There are enough Establishment Republicans who don't like Trump even without Mitt Romney.
McMullin, a Mormon BYU grad, worked on Romney's campaign in 2012 and the guy backing McMullin's campaign, John Kingston, is close to Romney, and was a major backer of Romney's 2012 campaign. Romney, Senator Mike Lee, McMullin, and other Mormons have been plotting to thwart Trump and deprive him a win in Utah. So as to screw up his assumed and necessary electoral votes. Sinister crap from The Despicables: Rick Wilson, Stuart Stevens, Mitt Romney, Sen. Mike Lee, Evan McMullin, et al.