For the second round of the SHA-3 competition, we decided to modify the parameters of Keccak. There are basically two changes: the modification of the rate and capacity values in the four fixed-output-length candidates for SHA-3 and the increase of the number of rounds in Keccak-f.
The increase in the rate was done for taking better advantage of the performance-security trade-offs that the Keccak sponge function allows.
The increase in the number of rounds is due to the distinguishers recently found by Jean-Philippe Aumasson and Willi Meier that work on reduced-round variants of Keccak-f up to 16 rounds. Although we think it is infeasible to exploit the 16-round distinguisher on Keccak-f when used in the sponge construction, we want the underlying permutation to have no structural distinguishers. This is the basis of our conservative design strategy: the hermetic sponge strategy (see the Keccak main document, Section 4.1.1).
Sticking to 18 rounds would not contradict this strategy but would leave a security margin of only 2 rounds against a distinguisher of Keccak-f. We think that the increase in the number of rounds actually increases the security margin with respect to distinguishers of and attacks against the Keccak sponge functions.
Finally, note that the modifications do not change the round function and therefore do not invalidate any past or ongoing cryptanalysis of Keccak.