As opposed to being conquered, collapse is typically caused by economic collapse; people stop obeying the law in order to get food, shelter, and what they need to survive.
The economic collapse can have multiple single causes, or a combination of them. Drought and subsequent crop failure (or unstoppable crop diseases like the Irish potato blight) can cause it; a drought of a few centuries long apparently had a lot to do with the collapse of the Mayan empire.
Rampant corruption and greed from the people in control can cause an economic collapse or societal collapse: In the latter, nobody trusts the law and takes the law into their own hands; or nobody trusts the money any more and refusal to accept it leads to a barter society.
Leaders of empires and their generals are typically less than 1% of their adult population: See here for modern America: How Many Elected Officials In the USA?
The answer: About 520,000, going all the way down to the local school district people and small town mayors and city councils. But there are about 245 million adults in the USA (with 74 million under 18), so 520K is less than 1/5th of 1%.
Now that doesn't include military or police, but the point is simple: All the government, military and police are overwhelmed about 100 to 1 by citizens, and this is true in an empire, too. If the leaders of the empire cannot keep their citizens fed, sheltered and out of desperation, the people will not necessarily revolt and establish a new government, they may just walk away and stop listening, and when needed respond with violence to coercion trying to take taxes or services.
That is what collapse without revolt looks like, people walking away, forming their own groups and breaking away, and in large extent the military and police just joining them and defending them. Because when it comes down to it (and I have been in the military) soldiers and police have to eat and feed their families too, and in an economic collapse they may join some co-op and contribute their efforts to that instead of the failed government.
I have read about collapse before, and cannot remember the details, but the solution was not something I thought of: people rallied around large farm and ranch owners for a fair share of the production and trade; agreeing to labor on the farm and protect the farm from predatory humans that would try to steal the food and/or kill the workers. The perfect job for cops and soldiers that don't want to join in with predatory and criminal gangs. Farm owners became basically mayors that dealt with other farms in trade for their products; potatoes for chickens or beef. People had to work hard on the farm because the collapse meant there was no power for the big equipment, they went back to farming, weeding, irrigating and harvesting by hand; not to mention squads that went hunting for the group to bring home wild food.
Added: I suppose the same idea, for a coastal area, might work for smaller fishing boats that could be operated manually. People need protection and have to work, the fisherman with help can supply a ready food supply but needs protection and muscle to get it all done.
The idea is that, in an economic collapse, most people revert to a "will-work-for-food" mindset. Of course some jump straight to a "will-kill-for-food" mindset, but the majority of us aren't capable and don't give ourselves much of a chance of prevailing with the latter mindset; so those willing to work will try to find a way to be useful to somebody that is already a food producer with the knowledge and experience (and perhaps some manually powered equipment) to do that successfully.