Pan Slavism was rampant.
On the Russian front, Czechs and Slovaks fled to the Russian side, and they even orgonised Czechoslovak legions.
Similar thing happened to Krajina, Vojvodina Serbs, and to some degree Croats too. Yugoslavic ideas were rampant in those days.
Much of this gave way to a lack of morale, lack of drive, and an awful amount of desertions from the Slavs, causing the armys performance to drop.