Well it was percolating once we got two newcomers at the same time and the count pushed up to four, but one was a clone and a popular request, and four wasn't seen as too egregious, so it remained in the undercurrent, a grumble here or there. Then we got Roy which pushed the count up to five, but he was a popular veteran, so the pressure rose a bit more but he was in turn excused and enjoyed.
Then the simmering pot of the bubbling discontent boiled over with Corrin, who, unlike the others, was not popular, not very requested, the fourth addition from a series in one game, promotional, wielding a sword, and last. Perfect storm material.
But they were smart about Chrom's reveal because they sandwiched him literally in the middle of a bunch of popular characters in the midst of a hype Direct, in the middle of the pre-release phase. He couldn't have been more cushioned if they tried. Which is basically the inverse of how Byleth was handled.