Its definitely happening everywhere.It's not only the diaspora community, I heard it's happening back home in Hargaysa as well. It's not a Somali women only thing, though. Across the globe, the number of single/unmarried women are increasing.
I think it has to with the gender roles, as more women are getting more educated and independent on their own they feel less need to depend on men, their standards are getting higher, it's hard to find someone compatible etc. The reasons are many, really.
I also think a lot of women choose to stay single because we feel happier that way. I say 'we' because I fall into this category.
But most women in their 30s usually want a child by that time, maybe not a relationship.