It's a Distrubuted Denial of Service attack. Where lots of different machines attack at the same moment, so they can't just ban a single IP.

They have happened for years (like even SB I think at least), often at routes leading to the server affecting specific ISPs. But SE got good at contacting those ISPs and setting up countermeasures instead of blaming customers.

So DDoS attacks stopped being very effective anymore until this one.