I had the same feeling as you when I watched the video again recently. It seemed like one of the ice giants would get ejected after coming too close to Jupiter. It turns out that there's a name for this: the [jumping Jupiter scenario](https://en.wikipedia.org/wiki/Jumping-Jupiter_scenario). Outside Wikipedia, it's described in [Fassett & Minton (2013)](http://www.nature.com/ngeo/journal/v6/n7/full/ngeo1841.html) (paywall!) and tangentially in [Deienno & Nesvorny (2014)](http://arxiv.org/pdf/1405.1880.pdf). Without this kind of encounter, the result of the Nice Model would not be what we see today (e.g. [Brasser et al. (2009)](http://www.aanda.org/articles/aa/abs/2009/44/aa12878-09/aa12878-09.html)).

This is what is known to the experts as a big problem. In most- but not all - jumping Jupiter simulations, the ice giant is ejected. One proposed solution is the [hypothetical 5th giant](https://en.wikipedia.org/wiki/Hypothetical_fifth_giant_planet), developed by [Nesvorny (2011)](http://arxiv.org/pdf/1109.2949v1.pdf). It involves a sort of sacrificial gas giant - possibly an ice giant - interacting with Jupiter and Saturn (as per the jumping Jupiter scenario) and then being ejected from the system.

The 5th giant planet would keep Uranus and Neptune where they went (i.e. not moving inward or being ejected) while giving itself up. I don't know why the simulation doesn't use that. It could be that it is one of the rare simulations in which the ice giants stay. But the Nice model either needs a 5th giant planet or some lucky initial conditions, unless I've missed something.