One reason the flat is on the market is pregnancy. People generally prefer a house to a flat when they start family.
The owner could have come into an inheritance or got a promotion and want something bigger.
They could be relocating for work.
Perhaps they are splitting up. When I was house hunting, I stopped asking why the vendors were selling because the response was always that they were splitting up.
They may have never lived in a flat before and miss having a garden.
The owner may actually have died and it’s being sold by family.
Of course, it could just be an absolute money pit and the people in the other flats are the neighbours from hell.