Quotes from the D3 Dev AMA

[Concillian]

If push comes to shove, you can always farm goblins.

Just a quick aside that I've been wondering about. When moving from normal to nightmare, did they make treasure goblins disproportionately harder to kill. I don't think I've been able to kill a single one since starting nightmare. Once he decides to pop his portal, nothing I do can dissuade him from jumping through. If I can't pump out enough damage, *giggle* *poof*. The best I can do is turn him around once, maybe twice, before he makes a portal but he inevitably poofs without giving daddy his yumyums.

You appear to have to do enough damage in a single hit to trigger a 'hit recovery' recoil animation. Doing any kind of CC will also do it (blind, freeze, confuse, etc...). In normal, they have low enough life that just about anything you do will trigger a 'hit recovery' recoil animation, which is perhaps the difference you're seeing as mob life increases faster than the damage you're doing.