Casting "quicken self"

[Quitschi]

I set my sc (tarrasque w1s10) to cast quicken self.
But instead it castes astral shield at the start of the combat , can someone out the tell me why the dragon ignores its orders? I already figured out that every S mage capable of casting astral shield does so as soon as he can, but why is the priority fr this spell so high? I mean it does almost nothing for a caster who stands somewhere behind in a army and even though my sc draw some use of the shield i would more like it to cast quicken self..