Angel investors have been around for a long time. But starting 2-4 years ago, the growth in wealth of successful entrepreneurs & investors means angels are proliferating at a geometric rate. They've also been able to become more organized, with angel groups like the Common Angels, and angel investor products like Angel List. At the same time, classic venture firms started making small, angel/seed-stage investments. Lots of investments, in fact.
The result has been really great in some ways: There is an amazing explosion of startups that are getting a chance at building a business.