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_AttackMaybe the amplification attacks however I am pretty sure most registrars don't attack each other for competition, it would be highly unprofessional especially in their field. Most of the money they make is from old domains anyways.