Cloudflare already mirrors Arch and Debian (https://cloudflaremirrors.com/) maybe they could help with Ubuntu too (might as well toss Fedora in there haha)
Edit: Facebook mirrors a lotta distros: https://mirror.facebook.net/ also I just realized it's a DDoS on the Canonical security API and ISOs not the packages, but I suppose those could be mirrored somehow
A DDoS like this, probably ransom. A group asking for x thousand or million dollars or they take you offline for a while. Another possibility is someone upset over Ubuntu's direction and is trying to "convince" them to do certain actions. Sadly, people are petty and greedy.