Technically, Nexstar doesn't control Mission because that would be illegal. However, they do share a substantial amount of services that doesn't constitute control (wink, wink, nudge, nudge)!
I actually wondered if some of the events at Mission were related to the announcement that Nexstar was putting itself on the block. The president and majority owner of Mission passed away a few months ago, and all indications are that his wife and son don't want to be in the business, though they are apparently willing to shell out a few million for another station. However, maybe Nexstar is shopping itself around because it knows Mission is doing the same thing? These pseudo-duopolies are central to Nexstar's business model.