Rockfreaks.net

...it's because their guitarist Bill Keiher was hospitalized in London last week:

"Bill Kelliher, guitarist with MASTODON, has been hospitalized in London as of Nov.3, during the band's current European Unholy Alliance Chapter 3 package tour. Doctors will continue to monitor his condition which is improving at press time.At the moment, Mastodon has decided to continue on the tour as a trio. Doctors and Bill are hopeful that he will rejoin his bandmates in time for their headlining tour of Europe which begins Nov 30, in Utrecht, Holland and will end on Dec 7, at the All Tomorrow�s Parties Festival in Somerset, England. More details to follow as they develop."