Is it really the registrars that are getting DDOS? Why would anyone want to go after a registrar, what benefit could they gain from shutting down so many small websites for businesses/etc?
Competing registrars.
or it could be DNS amplification attacks, where the attacker abuses the DNS system to direct traffic at the victim:
https://de.wikipedia.org/wiki/DNS_Amplification_AttackDNS amplification attacks involve a new mechanism that increased the amplification effect, using a much larger list of DNS servers than seen earlier. SNMP and NTP can also be exploited as reflector in an amplification attack.