Why is the iPhone's default landscape orientation 'upside down'?
I was going to chalk it up to the developer just being lazy and selecting the default value and tab-completing when creating their app. However, I tried it out for myself (create a new iOS app project, edit the $AppName-Info.plist file and add the following new row):

You can see that the first auto complete option for Landscape mode is (left home button), meaning the developer had to consciously choose the second option for it to be right sided as in your app. This supports stuffe's conjecture that this is done because most users are right handed.
The screenshot above are the options for iPad. The iPhone has the same set, except for the second one, which is absent.

AFAIK, Apple doesn't advocate the use of a specific orientation as a default. Their iOS Human Interface Guidelines only say the following re: orientation


Launch your app in your supported orientation, regardless of the current device orientation. For example, if your game or media-viewing application runs in landscape only, it’s appropriate to launch your app in landscape, even if the device is currently in portrait. This way, if people start your application in portrait, they know to rotate the device to landscape to view the content.

Support both variants of an orientation. For example, if your application runs only in landscape, people should be able to use it whether they’re holding the device with the Home button on the right or on the left. And, if people rotate the device 180 degrees while using your application, it’s best if you can respond by rotating your content 180 degrees.


On iPad, strive to satisfy users’ expectations by being able to run in all orientations. The large iPad screen mitigates people’s desire to rotate the device to landscape to “see more.” And, because people don’t pay much attention to the minimal frame of the device or the location of the Home button, they don’t view the device as having a default orientation. This lack of awareness of an app’s default orientation leads people to expect apps to run well in the device orientation they’re currently using. As much as possible, your application should encourage people to interact with iPad from any side by providing a great experience in all orientations.

So as you can see, they don't advocate for any particular orientation and encourage the developer to support all orientations. Also see this.
However, this technical documentation uses an example with code for right-oriented apps (i.e., home button on the right). So another explanation – apart from the developer's own UX inclinations – is a copy-paste effect :)