I may be overlooking something simple but I've recently built an action extension in iOS 8.1 b2 on Xcode 6.1 GM 2.
If I run the extension through Xcode (selecting the scheme and running; it asks me to choose an app to open the extension in which I select "Photos") and the extension runs as expected. But if I detach and run the photos app not connected to Xcode the extension seems to crash. If I try to attach to Photos already running I get a "lost connection" error in Xcode.
I'm not really sure how to debug as it only occurs if I start the extension outside of Xcode.
It seems my problem was related to high memory usage by the extension. For some reason, the system seems to kill extensions with high memory usage only when not attached to Xcode. The extension not only worked fine while attached but there's no indication that your memory usage is high (no warning it seems). Altering the extension to be less resource-hungry solved my problems as the crashes only occurred when the extension was handling a vast amount of data.