Why not just issue new cards?

I recently received a new debit card from my bank... Claiming they had 'detected a problem with my magnetic strip', which seemed spurious as I had never detected any problem with using the card. When this happened before, my old card continued to be okay until I activated the unsolicited replacement. This time, I was denied use of the card at two stores before I called my bank, who told me the card had been suspended - by the 'anti-fraud unit'. They didn't wait for me to confirm I received the new one by activating it.

I had indeed used that card at Target in the past three months.