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If you eat above your calorie maintenance level every day and don't use up those extra calories, it's very likely they go to fat stores.
But is that the only reason people get fat (being in a caloric surplus and not burning off the extra calories through physical activity)? Or is it possible for the body to still get fat even in a caloric maintenance or defecit in some other way (and how?)?
If you eat above a caloric surplus over an extended period of time you adipose tissues will expand to retain the caloric surplus. If you eat too much you will get "fat".
What defines a caloric surplus is different for every individual, some sedentary people can survive on 1800 Cal/day, some require much more. Furthermore, while you might see a chocolate bar has 200 Cal of energy this is under perfect laboratory conditions, so in reality you might take in less (but never more) of this caloric energy by virtue of the energy your body requires to digest the food. Unfortunately, nutrition is largely observational, and if you fix your intake at a specific number and and find you keep gaining weight you are eating too much.
There are very rare conditions where individuals are unable to release energy from fat deposits, however the only way for these fat deposits to grow is by having previously eaten a caloric surplus.
There are also hormones, drugs or chemicals that might cause your body to retain water. A well known example is creatine which (among other things) holds water in the muscles. However these cause body wide water weight gain, and you might look bigger, but it doesn't carry the same medical risks as excess adipose tissue so it doesn't count as making you "fat".
It depends on what you mean by "get fat".
If you are talking strictly about weight, then the only way that you put on weight is with a calorie surplus.
If you are talking about size, however, fat is much less dense than muscle, so it's possible to lose muscle and gain fat - and therefore size - even if you have a calorie deficit.
