Just saw a pretty big banlist for both historic (9 cards I think) and Explorer (4 cards) when my startup screen hiccup'ed.

Sounds like this was just a bug. Arena keeps a file on your device of which bans you've seen, so it can know which ones are new. This sounds like that file went missing or failed to load, and so it showed you all the bans (bolt, ritual, etc.) as if they were new.