I am considered a fraud

The other day, I tried to pay with my Mastercard at the supermarket. However, the display would just issue: “Bank rejected payment, call 044……..”. The lady at the cashier said that she had never seen this message before, so I was a little concerned.
Today I called up the hotline. I told the lady what had happened and she told me “ohh, we detected a suspicious transaction which we need to confirm. Have you on Jan, 3 used the card in Hawaii for a sum of 15USD?” I told her yes, why? (I bought Internet access for our hotel room with that) She said, “OK, that’s all, you can continue to use your card now.”

I am really happy it was nothing serious, and I am glad to see that credit card companies actively identifying fraudulent activity, but hey, they do have my full address and phone number. Can’t they call me up and ask me instead of just closing down my card and waiting until I would call their hotline? I could have needed the card for something serious.

Disabling the card because of one payment of 15USD? I wonder what algorithm they use to detect fraud.