Collaborations have been happening for decades. Those of you who are fans of pop and rock music might find them a bit weird, because in those genres, artists fend for themselves. They whine when people cover their songs, they're constantly in competition with each other, and are happy when their competitors fail, and upset when they have success. If someone wants to sample them, they're often selfish and take them to court, or demand tens of thousands of pounds from them.
In urban music, this is not a case. Hip hop, for example, is more than just a genre of music. It's a community; a lifestyle. Most rappers are completely fine with making a song, and then dozens of other rappers adding verses on without even asking. A lot of rappers can't sing (very well), so they often get proper vocalists to sing the choruses for them, or they might sample the chorus from one of their friends songs, if it is appropriate. It's completely different there. Instead of "Don't sample my songs, and if you do, I'll take you to court and I'll order you to give me lots of money" like some of those rock artists try to do, it's "I made this song. Everybody feel free to do your own interpretations of it".
Seriously, it's a completely different world. Collaborations are perfectly normal in hip hop, RnB, etc. The artists in the mainstream are often really good friends. Some even live together. The more experienced rappers tutor the less experienced experienced rappers, and help them to get played on the radio, and so on.
Some artists aren't selfish and obsessed with money, fame, etc. anymore. This may be a shock to some people, but a lot of artists, urban artists in particular, like to SHARE.