Since so many vocal nevertrump seem to be Mormon, can you tell us more about what they are told?
Why do you assume they were told anything? The Church never endorses a political candidate. Politics is never preached from the pulpit ever.
Extermination Order
All Mormons know about this. And Trump proclaiming to focus on a religion for discrimination is a slippery slope which probably raised a red flag among many Mormons.
In 1844, when Boggs signed the Extermination Order, mobs went out killing people. My wife's ancestor, great-great-great grandfather was gunned down by a mob that was lead by three protestant ministers. He was out working in his garden, because he was a Mormon.
So yea, we're a little sensitive about that subject. And why many Mormons would say "never trump", is understandable.