i think you have it backwards...rappers are just trying to do it themselves and feeling more comfortable singing than they have in the past. people have always been into sung hooks though...its not new.

if method man had tried singing in like 95 people would've called him a sellout or soft...thats not the case anymore so more people feel alright doing it themselves. its what 50 dissed ja rule for doing even though he went on to do it himself moments later.