The origin of the term "writing on the wall" finds its roots in the Holy Bible. The painting above, done by Rembrandt, illustrates the scene in which Belshazzar is being informed of the impending doom that will overrun his kingdom, Babylon, for utilizing stolen material from the Jewish temple during a feast. What the phrase "writing on the wall" means should be obvious to any person with a modicum of knowledge concerning the English lexicon; practically, something that is bound to happen or something negative that the subject learns is virtually inevitable.
So with that in mind, the topic of evolution really is a giant bugaboo for the theist, most notably in the monotheisms which state that God takes more than a passing interest in the affairs of mere human beings and that our trifling occurrences on the middling planet referred to as "Earth" is the apotheosis of the universe. The problem of course is the contradictory nature between Scripture's account of man's creation and the scientific account of man's emergence. Now, with this recent scientific discovery which, surprise surprise, does not match up with an ancient and naive guestimate as to how human beings developed, theistic apologetics are forced into conceding the point that evolution has happened and is about as reasonably considered a fact as you can derive from science. So much so that detractors are compared to flat-earthers and Holocaust deniers.
Let's keep in mind now that even though St. Augstine theorized that God is not limited to literally creating the universe in a matter of six days and that this interpretation could possibly be purely metaphorical, the young Earth creationist theory was the default position and taken literally for quite some time, including many in and around the time of Charles Darwin. The peculiar case for theists now is to explain how evolution maintains a certain level of efficiency, not during the course of evolution as evolutionary progress more than likely tends toward more efficient mechanisms to interact with the environment, but on the part of God's creation.
The sticky predicament for theistic apologetics is to explain why God chose this route for human beings to develop and "prosper." The amount of time, for one, seems exorbitant and wasteful. As we can see in the process of domestication and selective breeding, known "intelligent design" used by humans can produce massive genetic changes in a few thousand years. The cow is a great example of rapid evolutionary change thanks to intelligence. Now, assuming nature operates via blind and indiscriminate mutations, no wonder it takes so long for species to differentiate and evolve. It appears as if there was no intelligence behind the course of evolution when we compare nature's inefficiency and intelligence's swiftness at generating change in organisms' gene pools.
But that aside, there is nothing stopping God from snapping his fingers and creating the state of the universe as it is. The amount of death and suffering that heartless evolution creates seems excessive in the least and there is no conceivable reason why this alternative is more efficacious than my finger-snapping scenario. Christian apologist William Lane Craig has said in a debate versus Austin Dacey that it is "awfully presumptuous" to assume that if God does not meet our standards that he therefore is either capricious or non-existent. But what other standard are we allowed to use? After all, we can't play this both ways when we assume that God has given us divine insight into the key features of reality and then turn around and say that our perspectives are puny, puerile, and faulty.
Perhaps even worse, this same apologist has made the case that efficiency only matters to those with "limited time and limited resources." Exactly, which is why it makes God's design apparently even worse! Maybe if God was constrained by certain elements that this creation was the best he could compile, but really? This disavows God of his ability to make a more efficient creation? If he is not constrained by these elements, then I believe we have every right to expect better than to be ridden with cancer, not being able to grow back severed limbs like many amphibians, be without sonar like bats, have better immune systems, and maybe even be able to fly manually (hey I can dream, can't I?). Of course, these features would just be my subjective preference concerning what I would find to be cool, but many of us can agree that these physical attributes can be an improvement. We don't even have to ask for much else, just that maybe we could live healthier lives and that disease were not so prevalent. Or how some kids may not grow up to be autistic. Or how some women wouldn't die in childbirth.
So in conclusion, I believe it seems that the reason theists are so pestered by evolution is that the writing is on the wall so to speak. The Belshazzar in this instance is the reluctant, squeamish theist while the mysterious hand may be scientific inquiry and rationality concerning the outcome of understanding evolution. What the theist fears is that this dislodges us from our perceived place in the cosmos and makes us one of many species struggling to survive in a chaotic and unreasonable world.