These are corrupted directory entries, stored in the File Allocation Table if the card was formatted for an FAT family filesystem. Reformatting gives you a new clean File Allocation Table, and therefore the spurious entries disappear.
Thanks to Wikipedia for providing this background:
The name of the file system originates from the file system's prominent usage of an index table, the File Allocation Table (FAT), statically allocated at the time of formatting. The table contains entries for each cluster, a contiguous area of disk storage. Each entry contains either the number of the next cluster in the file, or else a marker indicating end of file, unused disk space, or special reserved areas of the disk. The root directory of the disk contains the number of the first cluster of each file in that directory; the operating system can then traverse the FAT table, looking up the cluster number of each successive part of the disk file as a cluster chain until the end of the file is reached. In much the same way, sub-directories are implemented as special files containing the directory entries of their respective files.