I have found that it is very easy to "purchase" an Amazon Prime membership without intending to.
It seems that if they have your credit card details on file, and you are putting items in your shopping cart, they have the process triggered and without even checking out, a notification is received stating that you now have Prime membership.
I know you can cancel it, but what a PIA, and opportunistic practice.
I had same thing happen, some how auto added it when checking out .. buggers.
i wen't in and cancelled it but given I've had my free month if it happens again it will likely charge me and it will be the last time I use Amazon