Can't speak for US banks as I am from the UK but I worked for a bank for 5 years so maybe this will help.
In the UK if you make a transaction that looks like suspicious activity on your account then the fraud prevention section will sometimes call you. Now don't take suspicious activity to mean "oh shit that boy is buying seeds BUST HIM!!!!" Basically what it means if the activity is suspicious for your account. So if you lived in america and then went on holiday to france and started spending money on your card they might call you and be like "Hey are you in france or has some french loon stolen your card?"
Basically they are calling you to protect you and themselves from fraudsters. Your transaction probably got flagged because you are spending money in a uk based business and that's something you've probably never done on your account before which is why it set off alarms bells. You could of been buying cheese or a chess set and they still would have called you. Or the DEA are on to you man....