I'm guessing that the SD card doesn't sit in the card slot and recessed area very well. Sometimes, over time, the card might be jostled by handling of the phone like if it gets bumped. The contacts lose a solid connection with the other half of the contacts inside the phone and an error happens. In some cases with some phones, I speculate when a card shift happens during a write cycle to it, the file allocation table doesn't get updated correctly, thus temporarily corrupting the card. Seems as if a format fixes it sometimes. That's my guess. When I put a piece of thick paper, less than a millimeter in thickness, on top of the card, and the paper is the same shape of the card, and the weight of the battery on it helps keep the card from having the tendency to move around in the card socket. The errors don't seem to happen again for me.
Why does this crap happen to me? The spring loaded card sockets on phones, I've never had this problem. They seem to hold the card in place very well. It's probably a cost issue because a plastic bar to hold the card down is cheaper than the metal spring loaded locking card mechanism.