While an ISP maintenance activity may have played a factor, the real problem was that Border Gateway Protocol (BGP) routing tables have grown too large for some top-level Internet routers to handle. The result was that these routers could no longer properly handle Internet traffic.
BGP is the routing protocol used to share the master routes, or map, of the Internet. On top of this the Domain Name System (DNS) is layered so that when you click on “www.zdnet.com” you’re taken to ZDNet.
When the BGP maps grow too large for their routers’ memory then, as the Internet Storm Center said, “BGP is flapping.”