I believe that it is refered to the cancel method of the Lineage 2 (oh no, an archer comes! Press ESC and run!)
Fix your test map (Trigger, which forces Priest to Throw thunder bolt doesn't work

)
Annd uhh.. your spell's are in quite weird setup line (QEW, note QWE). You can
a) Change their sequence in object editor
b) Change the Hotkeys and tooltips
Oh, and BTW, when attacked, Caster could lose only a part of casting (like-- 25% of all casted time), not fall to 0, as very fast normal attacks would be totaly imba against caster-heroes
EDIT: Interesting... WHen the test hero was Mountain King, the casting time falling was to 0, but now it isn't

Aaaand... Note that not all heroes channeling animation is 'Spell Channel'. Some use 'stand channel' (Like Paladin or Archmage). Also, most units don't have such an animation, so they would simply stand and do nothing (like Blademaster). You could make a setup part to get different animations (string variables) for every unit tipe. In it people would simply write their animation names for casting like 'spell channel' or 'stand channel' or even 'stand ready'
Moreover, there is a little noticable time until the caster starts playing the Channeling animation.
I wonder, could that time above the Cast bar (Near the name of the Spell) decrease as well as the the cast bar fills?