-Civic costs increase as your total empire population grows
-City maintenance scales based on population of the city
-Adding more cities also scales maintenance in all cities unless you are already at the difficulty's cap (i.e. 8g is the "per city" cap on Deity, paid in all cities)
So your expenses will just naturally increase as you expand and grow your cities, which happens organically as you tech and/or trade for more happiness and health.
Inflation is also an expense that factors, and it scales based off your current total expenses and is incremented by the turn. So the later the game goes, the more you pay in inflation, making things gradually more expensive just by advancing turns as well.
Also note that the civics you run have different costs. Some of them can be very expensive depending on your empire, while others can be live-savingly cheap.
Expensive civics:
Police State
Bureaucracy (with many cities)
Organized Religion
Mercantilism (you lose all foreign trade routes, which is a major commerce hit!)
Vassalage
Cheap civics:
most of the default civics such as Paganism
Bureaucracy (can pay for itself and then some if you are small and run a cottage-capitol)
Nationalism (especially in big empires)
Pacifism
Free Religion
Free Market (with no corps around the +1 trade route is very good)
State Property (the mack-daddy of cheap civics -- if you are not very small)
Then there are several dynamic factors specific to any individual game, which are not always in play but they can happen:
-Difficulty level modifies the static "per city" cap on maintenance; the cap is higher on harder difficulties, making any new city cost your empire more.
-City distance from the palace increases maintenance unless in State Property civic, so as you naturally add more cities away from your capitol as the game goes on (logically you'll expand to closer spots first in general and then continue on assuming there is still land) those cities will be more expensive for the empire
-Cities on distinct continents start paying additional "colonial expense" maintenance once there are two or more cities on another continent from the palace, and it scales with each new city
-Corporations, if present at all in any city, cost the city's owner expenses in the form of corporation payments. Doesn't matter if the corp is yours, or if the AI spreads it to you (do they even do that? I never see it), the presence of the corp costs gold unless you are in State Property
-Trade route shenanigans. AIs love to adopt Mercantilism once they start teching Banking, which creates a window of the game where they shut off all trade route income to you when they use the civic. Since foreign routes are worth more commerce, it can start to strain your economy if you are trying to pay expenses with only domestic commerce sources.
-There can also be shifts in trade route dynamics based on diplomacy and empire sizes: If you go to war with someone, or stop trading (due to demand), you lose open borders and routes; AIs will also more rarely close borders at random when they dislike you. If your empire starts getting bigger than most others, more of your routes will start becoming domestic as foreign cities can only ever trade with one other foreign city, but there can be any number of domestic routes to accommodate the number of routes per city you currently have unlocked.
-An even rarer scenario is an AI having all their foreign routes going to other AI's cities. Since trade routes automatically calculate based on the highest potential value when selecting target cites (based on size, distance, being to a foreign city, and things like being on another continent or sustained peace) this would only come up realistically if other AIs were all bigger than you but their cities were also larger and further away from each other than yours, and in enough number to suck up all the trade routes first...something which could actually happen on harder difficulties like Deity, though generally there are so many routes per city this is unlikely to happen, and if you are forced into this scencario you would be small enough to not really be relying on trade route income to fight expenses