Debit card replaced

• http://www.theguardian.com/money/2012/feb/03/barclays-replace-debit-card
• http://www.help.barclays.co.uk/faq/cards/debit-card/why-new-card.html

Barclays sent me a new debit card out of the blue, my existing one still had 18 months until it expired. I phoned to find out why and initially the guy said, yes, strange, let me check for you. Then couldn't find the reason and said probably just because the system had detected the old one was getting worn out. But that didn't ring true, I hadn't had any problems with it at all. Then eventually he said he'd found a note against the new card that it had been replaced due to fraud. But that didn't make sense either. They always block my card and ring me to verify transactions when they suspect any fraud. But they didn't block my card - I'd used it several times that day as well as previous days - and as far as I can tell, they didn't try to contact me either. It went back and forth for a while, me trying to get a better explanation out of him, he not having anything else to offer. Eventually I just cancelled both the old and the new card.

The couple of links above seem to suggest it probably was to do with fraud. But still, why wasn't the compromised card blocked?