First of all, it says right in the blog post they believe it was a state-sponsored group in China:

Secondly, notepad++ is software. Software is not always written perfectly first go-round, so there may need to be updates made to the code. Rather than the developer going around to everyone’s houses with a USB stick, we make use of “the internet” to deliver those updates. For convenience, software updates are often automatic, with little to no user intervention required.

I hope that clears things up.