We’ve discussed this before:
- fiscal inability;
- an elite faction which wants to take over;
- support of 20% or more of the population;
What is also very common is lost of prestige due to military defeat. This Russia in WWI (and they had also lost to the Japanese, which was particularly humiliating) or Germany, also after WWI. Getting your ass kicked makes people doubt you.
Or, if you want to put it another way: imperial overstretch, military defeat and internal strife.