The two design patents, both showing drawings of the original Apple iPhone, are being challenged because of three prior art drawings that were patented in Japan. These three Japanese patents weren't considered when Apple first applied for its patent, but the anonymous party who requested the re-examination says that the similarities are too obvious. The USPTO has not yet rejected Apple's claims to either patent, but has agreed to take another look at the pair.
The third patent is being used by Apple in cases against Samsung and Motorola. Apple's U.S. Patent No. 8,014,760 for missed call management, has five prior art challenges against it and the USPTO has agreed to review that patent as well.
While the anonymous challengers are by definition not revealed, it is believed that Samsung is the party that asked for the re-examination on the two design patents while the third challenger is truly unknown.
source: FOSSPatents via AppleInsider