For Darwin, one can look at the Scopes trial of 1925. Whatever actually happened, the trial concerned the teaching of human evolution over the Biblical creation myth. Religious fundamentalists opposed the teaching of evolution as they saw it as going against their faith.

Marxist link. Marxists in the early 20th century came from a workers' movement. With the withdrawal of the Russian Empire from WW1 and the subsequent revolutions leading to the USSR, plus the labor movement, Marxism was then seen as a threat to the status quo of the rich, industrialists, and capitalists.

For Freud,it has been said that he viewed his own work as a challenge to human vanity, similar to that done by other scientists. It certainly challenged the typical perception of psychology and how it worked, and once again, those who were taught the older ideas opposed them. A strong focus on sexuality would not have helped either.

For all three of these sets of ideas, there is also the general threat to tradition or the way people believed things were or ought to be. Tradition is a very strong influence itself, and especially religious tradition which many people saw as threatened by radical new ideas.

*edit for formatting.