There were a number of "situations" that did this. Let us look at two of them.
First, there was the assassination of President Lincoln. Lincoln was inclined to be lenient with the South. At the same time, he had enormous prestige in the North and the Republican Party. Had he lived, he might have been able to push the South to change in some important, but more moderate ways without making the Radical Republicans angry.
Second, there was the intransigence of the Southern governments set up right after the war. The creation of the "black codes" for instance, showed that the South had not "learned its lesson" from the war. This angered the Radical Republicans and set the stage for more drastic actions that caused Reconstruction to be acrimonious and ultimately unsuccessful.