Three possibilities come to mind:

1) There is some fundamental data aspect Kiwix hasn't mentioned (or is entirely unaware of). I.e. CP or some other super illegal stuff.

2) Hetzner is profoundly incompetent, deleted production servers by accident, and the "But we sent you an email!" thing is a lie to cover up the mistake.

3) There is some kind of interaction that happened prior to this that we aren't privy to. Perhaps a series of late bills, legal threats, or some other inter-personal issue.

Predicted outcome:

I either expect Kiwix to get a knock by federal/national authorities. Or the more likely outcome in my opinion: some frustratingly vague statement by Hetzner PR about its customers being "mistaken" in regards as to why data-go-poof.

I mean seriously, let us assume it's something illegal: Sure, fine, whatever. Wouldn't it make more sense for that material to not be deleted, so whoever the guilty party is would be arrested for/prosecuted by it? Deleting the servers would be like police being informed about a murder weapon and asking the tipster to destroy the weapon before an arrest is even made. It doesn't make any sense to me. Surely if some bad thing were discovered, there would be some method to encrypt/restrict illicit material without destroying it.

Either bad blood, or unpaid bills, or simple incompetence seems like the most likely culprits to me.