Why is rumble one shotting with ultimate?

If you play ADC, you'll understand.

A Rumble, who ISN'T full build, spam clicks R on you every team fight. And you die to it.

You don't even have to stand in it to die, as long as it taps you and you walk away the burn will kill you 100 - 0.

It's so bull shit. Yes I built Hexdrinker and Mercs.