Alright Boards, explain this.
Im waiting. [zombie-nunu-bummed]
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But seriously, wtf happened there?
You poly'd Ez as he was ulting. I had this same issue playing Tahm Kench when eating Ez while he was ulting. It's something weird with Ezreal because once he starts casting his ult there's no way to stop it except from a stun I think, and he'll continue casting in a polymorphed form or in Tahm's stomach.
Much like 
Final Spark, when the ultimate is casting it cannot be stopped. There's info on this on the LoL wikia.
He must've 'buffered' ulti probably like a frame before your polymorph.