Unity has a feature to recompile scripts on the fly while being in the editor called live recompile. When changing a script while the unity editor is in play mode the scripts will be compiled again and tries to setup the scene again.
The Vuforia Play Mode will stop working when live recompile is attempted. After that when starting the play mode in the editor, Vuforia will fail to initialize and only starts working again after restarting the unity editor fully.
I don't really want to use the live recompile feature. It just so happens that I accidently use it when I change code without stopping Unity which will then also break Vuforia.