The same thing happened to me. I had earned numerous $25 VISA gift cards. Over time, I would add each card to one of my PayPal accounts, and then send the money to my other PayPal account (and then withdraw to my checking account). That's when they froze my account, telling me that I had reached my lifetime limit of adding 16 credit cards to my PayPal account (although I would delete the card after I had used it).
They unfroze my account, but they told me to knock it off.