Why is ezreal allowed to shift out of my veigar cage while stunned?

There really should be a sticky at the top explaining the difference between cast animations, channels, and dashes and how CC effects (or doesn't) each of them so we don't have the weekly Ez E/R, Lux R, Trist W is bugged thread.

You stunned him to late. Once he starts the cast of all his abilities you can't stop them. His E and R just have really long cast times.