Theft, mainly. A transformer is a big lump of copper, but even the oil is meaningfully valuable in a low-income country. There are reports of widespread transformer oil theft across Africa and South-East Asia. Draining a transformer of oil is easy, it'll earn you a few naira and it'll inevitably lead to the transformer burning itself out in spectacular fashion.

https://www.aljazeera.com/indepth/features/2014/12/thieves-f...

http://www.eskom.co.za/AboutElectricity/PubSafety/Pages/Tran...

https://english.mathrubhumi.com/news/kerala/theft-of-oil-fro...

Could be any number of causes:

- overload + faulty breakers (or no breakers...)

- old age insulation failure

- coolant depletion

- overvoltage due to lightning strikes elsewhere