There is a lot of information (including links to the original CAIDA studies) about negative DNS available at AS112.net
If you know how BGP (it's an IETF standard that makes the Internet work by routing autonomous systems, or ASes, through a path vector routing protocol) and DNS (it's another IETF standard that makes the Internet work by allowing DNS servers to access a root name server infrastructure) works, then you understand that there are neutral operators out there that run the infrastructure that keeps the Internet going.
As you can see from the CAIDA research, RFC1918 DNS PTR records started to leak out to the global Internet around 1997. The root name servers could not handle the load of this extra traffic, even with the a more advanced Anycast setup when BGP announcements of more-specific /24 IPv4 prefixes were utilized with a root name server on a /32 IBGP announcement.
An entire AS was created to handle the extra load of this traffic, and it also utilized Anycast. Organizations are probably slamming their own outbound firewall rules with this DNS traffic, affecting their performance as well (especially in huge egress outlets). The autonomous system for this negative DNS traffic is AS 112. It's website (and more information about the history and statistics of the growth of this negative DNS traffic is hosted there).