Rijndael was the only AES candidate which defined a different number of rounds for their 128, 192, and 256-bit versions (10, 12, and 14, respectively). The others had a fixed number of rounds (32 for Serpent, 16 for Twofish, etc.) regardless of the key size. Why was this? It may have reflected the intention of having three different security levels and leaning towards speed for the "lower" security level, but that still doesn't explain why only Rijndael did this instead of settling on, say, 14 rounds.
Is there some cryptographic attack which is unique to Rijndael which would warrant this?