Answer:
They took the Holy land from the Christians and that they were starting to take over the Byzantines.
Step-by-step explanation:
The Seljuk Turks took over the city of Jerusalem, where Jesus was born and which was a big deal to Christians. In addition, the Byzantine empire needed help from Western Europe or it would be defeated by the Seljuk Turks which then could cause all of Christian European civilization to be threatened by the Turks.