Question

why might the action of the seljuks in conquering southwest asia have ignited the christian crusades

Answer 1

The Seljuk's control of the Middle East sparked the crusade not only because they followed a different religion than that of Christian Europe, but that their growing power threatened to spill conquest over into Europe itself. The Byzantine Empire, seen as the last defense of Europe against the encroaching armies of the Seljuk Turks, was also slowly weakening. Something had to be done.

Christian Europe was then called to arms by the Pope who first sought to support the Byzantines in their time of need, but later expanded it into fighting to reclaim the Holy Land itself from the hands of so-called "infidels."