When iOS 8 was unleashed on the general public, the size of the update — 4.6 GB — caused many users' iPhones to cower in fear and shame. In an attempt to avoid a repeat of the problem, iOS 9 will free up space by deleting third-party apps before downloading (and then reinstalling afterwards).
The feature — if deleting a bunch of your apps can be considered a feature! — was discovered by a user on the MacRumors forum, who was alerted to the auto-delete functionality when trying to update to the latest version of the iOS 9 beta. According to the pop-up they received, all deleted apps would be redownloaded immediately after the update — although, it's not made clear what happens to your data contained within those apps, something Apple should clarify pronto.
Apple is clearly trying to avoid a repeat of the iOS 8 update debacle, where a combination of early bugs and the 6GB of free space needed meant that users were slow to download the new software. iOS 9 should already be much better: the update only requires 1.3GB of free space, so just delete a couple hundred of those selfies (or let Apple do that for you) and you should be home free.