Africans, buses, and seat sharing: cultural phenomenon?

I use the bus a lot, and I started picking up on an odd phenomenon. It appeared to me that recently-immigrated Africans don’t like to share their seats on the bus with strangers. I wondered at first if it was some kind of latent racism on my part, so I started watching for it specifically and was able to confirm my initial observation. It doesn’t seem to matter whether they’re young or old, male or female, poor or moneyed; they seem disproportionately likely to not share their seats.

It got me thinking about why this should be. Is it a cultural thing? Is it a result of simply being a recent immigrant and not wanting to rub shoulders and hips with strangers from a different ethos? Is it a local thing? Or am I simply imagining all of this out of some kind of subconscious racism? Has anyone else noticed this? I once had a Pakistani friend who told me that North Americans all smell like spoiled milk to him (presumably from our dairy-rich diet) and was quite surprised (and insulted) when I told him that he carried an aroma of curry from his own diet. Could it be that recent African immigrants don’t like our smell?