Why was the house wiggling?

The above was my three-year-old's response to this, which we experienced at my friend's third floor apartment in central Wellington last night (screengrabs from the awesome Geonet, click to enlarge):

Here is what it looked like on the local quake drum:
After living here for a while you tend to brush off the small ones. A few seconds of shaking and it's back to whatever you were doing, having spent at least the first one or two seconds deciding if you needed to take shelter in a doorway or under furniture (the bigger shakes tend to make that decision an instant one I have noticed). This one got worse after a few seconds, and when they do that is when you really take notice, usually by wondering "Is this IT, the big one?" in my case.

And then when it stops there is always the period where you are wondering if things are still moving or is it just the adrenalin making you think they are. This was the biggest shake for a while around here.