why considering in what numbers Czhes ran over to Russians
I do not think there was mass desertion by Czech soldiers in the Austrian Army.
Rather, large numbers of Auustrian soldiers, including many ethnic Czechs, were taken prisoner during the fighting with the Russian Army, particularly during the first Russian offensive in 1914, which advanced deep into Austrian territory.
Then, Czech political leaders from the United States came to Russia, and persuaded the Czech POWs to join a force to fight on the Allied side. That is how the famous Czech Legion was born.