Snow Was Too Slow To Grasp What Katniss Was Igniting
Prior to the 75th, Katniss and Peeta are given cards to read which they choose to ignore and basically speak out their hearts. After the chaos it causes, they instead choose to read the cards instead, to prevent further bloodshed, the thing that causes the rebellion in my opinion is not the monotone way they read the cards but the fact that they tarnished the first impression that Snow wanted them to portray to the masses which was that they were authentic friends rather than pretending to be which was what the monotone and sudden shift to reading the cards from reading their hearts out.
If I was Snow, I would have immediately met with Katniss and Peeta altogether, the day immediately after their first tour in District 11 that led to chaos and told them to instantly cut it off. If he truly had control of the situation, he would've understood that the next step they would take in an effort to calm the Districts and prevent Snow from getting angry further would be to read off the cards which is just foolish and a display of forced speeches.
Snow was too slow and is the key reason behind his downfall, its pretty obvious that he is, but this is one of the first few causes in my opinion other than Seneca Crane being an idiot and basically giving both Peeta and Katniss full control on how they wanted to end the 74th Hunger Games.