This happened to one of my mates:
1. Ordered some stuff from Amazon using Tor
2. Amazon tracked the change in IP
3. Amazon automatically cancelled the orders and sent a password reset email saying his account may have been compromised
Sounds like bad OPSec. If he used his normal account via Tor he identified himself towards Amazon. Amazon checking against a list of Tor exit nodes and warning customers is a good thing. I say that as a Tor lover.