It's always crypto. A client got some AWS credentials stolen and without anyone checking the account, the hacker managed to spin up big EC2 instances across many regions. The bill after a month as I recall was around 100K. Since the activity was clearly fraudulent the bill was forgiven eventually. So remember to lock down your AWS keys permissions...
That; and also, enable the various monitoring and audit features in AWS now; start with CloudTrail. Nothing worse than being affected by this attack, and AWS not having any audit trail available.
When that happened to a former employer AWS was calling us within a day. Worth making sure a real phone number is on there, as that's how they contact you for anything serious (and also if your finance dept decided to change the credit card without telling anyone)