The IOC World List mostly followed the classification and sequence of families in the third edition of the Howard and Moore Complete Checklist of the Birds of the World (Dickinson 2003). That classification reflected the phylogenies based on DNA data available at that time, summarized in the chapter on “Avian Higher-Level Phylogenetics” by Cracraft, Barker, and Cibois.
We now have the ability to to distinguish taxa of similar rank that are reciprocally monophyletic. This lets us redefine some groups of species that in hindsight were paraphyletic. Changes in taxonomy follow, especially realigning of orders, families, and genera to define monophyletic taxa that can be reconnected to their closest relatives in the classification. Genera also change with corrections of nomenclature and the rules of priority. As a result, we can expect ongoing instability of working classifications for a decade or so. Continually improving sets of DNA sequences and analytical approaches may override previous conclusions, so cautious revisions of higher category taxonomy and new sequences of taxa are prudent.
Excellent recent resources include Joel Cracraft’s (2013) Avian higher level relationships and classification in Howard & Moore 4, Jon Fjeldsa’s (2013) Avian Classification in Flux in HBW Special Volume and John Boyd’s website Taxonomy in Flux. For comprehensive views of the relationships among Orders, see Rick Prum et al’s (2015) A comprehensive phylogeny of birds (Aves) using targeted next generation DNA sequencing, and Erich Jarvis et al’s (2014) Whole genome analyses resolve early branches in the tree of life of modern birds.