Question

Why might the action of the Seljuks in conquering Southwest Asia have ignited the Christian Crusades?

Answer 1

The really short answer is that it made Christians in the West really upset that the Holy Lands were now in the hands of "infidels" (essentially anyone who wasn't Christian). The Byzantines were also unable to stem the tide by themselves opening the door for the West to step in with a call for the Crusades.

But economically, many convenient trade routes to the rich silks and spices of the East also ran through territories now controlled by the Seljuks -- routes that now required things like heavy tolls and taxes to be paid.