^A posting to the tzdata-history mailing list provides original documents: http://groups.google.com/group/tzdata-history/browse_thread/thread/5e0d0d24ba438e4c
In short: A US government decision in March 1918 announced a change in time zone boundaries. The CT/MT boundary was to run through Texas roughly along the meridian 100w, with a bulge to the west around the towns of Sweatwater, Big Springs and San Angelo, starting January 1, 1919 at 2 am. The local Panhandle and Plains chamber of commerce was not happy in 1919 to be in another time zone than the more populated south-east of the state, and they petitioned a change.
A US Congress decision of March 4, 1921, became part of the United States Code as section 265 and moved the Panhandle and Plains area back to CT.