Sounds like the Ezreal jumping out of Blitzcrank grabs deal.
Pretty much no ability casts the instant you hit a button - there's a portion of the skill's animation that needs to play before the business happens. And then usually a fallow-up animation after the business happens, which can be interrupted with no consequence for the purposes of orb-walking or kiting in several cases.
But having a brief period of time (usually in the 0.5 second zone) where an ability is cast but doesn't do the abilities things yet leads to a lot of strange interactions like the one you're describing.
Some champions can get cheated out of their ult by being killed during the wind-up animation phase: having their ult placed on CD and being drained of mana without any ult actually happening.
With how several dashes have been tweaked currently - many champions can get hit mid-dash and don't get inturupted during their dash allowing them to reach full duration, but they incur any CC they've been tagged with once they land.
It's more noticeable in Dominion as action is non-stop and you're in 1v1 or 2v2 fights more often than team-fight chaos so you can actually see these shinanigans going on.
Lag/latency can also be a culprit for what you described.
Anyways, hope that at least explains what's going on. The particular interaction you described probably isn't a bug considering how Tristana's rocket jump has been changed to do a similar thing on purpose - but such an interaction is still awkward enough it could warrant posting on the bug report forums for some feedback.
Several shenanigans ult-CD robbings I've experienced in the past have been patched to refund mana and their CD if they don't even fire.
vs 
midlane. After we got our ultimates I noticed that Kass is a really good counter against Yasuo, what with me just teleporting out of the way of his tornados.
