My credit card no. was being mis-used?

This afternoon, I received a call from Citibank telling me that the bank suspects my card was being used misappropriately by someone. For security reason (or i think should be due to the bank's risk management process), they need to void my card immediately and would issue a new card to me with a different set of numbers soon. The new card would be sent to my home by courier.......oh well....as far as i remember, this time is already the 2nd time i have such problem with my Citibank credit card.....last time was after my honeymoon trip dated back around Nov/Dec in 2005.

I have a big "?" in my head....how can a bank know that my a/c was bring used inappropriately? What are the transactions done by those thieves with my card?? Can anyone tell me?