I was talking to my friend at work about this as my
Neighbours are Nigerian and they are always loud.
It's basically demonstrating that you have nothing to hide as in Nigerian culture the quieter you are, the more likely you are shady/shifty. So it's aoat inherent and seems aggressive but it's just confidence.
Noticed this too so I asked my Nigerian friend and he was like it's because they use mostly Generators in Nigeria so it's affected some people's hearing or something and they just raise their voices naturally so as to be heard, and on going to Lagos last year I confirmed how true this is.