You raised some red flags with the information you provided. This doesn't happen to everyone. A support rep from Hetzner has spoke a bit more about this process on WebHostingTalk before[1], although they don't get into which specific heuristics may result in flagged accounts for obvious reasons. I'd imagine it's a combination of things like unpaid balances on previous accounts, IP address reputation, uncommon e-mail domains and so on.

[1] https://www.webhostingtalk.com/showthread.php?t=1810197&p=10...