The map is decided by voting and the voting UI is broken in that the first map it shows for nominating is TE-Barracks and to see the others you have to scroll down. Same applies for voting for maps
But it constantly resets the scroll bars so you can't scroll down
So people start clicking on stuff and nominating it and voting for it without really understanding what's going on
I left a little more detailed post on the official steam forums that elaborates a little more on this and other issues I've found.
Servers I've playing on change maps every few rounds but no wonder Barracks is played a lot, it's terrific.