In this post, Alex Golesh talks about tombstoning – the part of an application lifecycle on Windows Phone 7.
The Windows Phone execution model governs the life cycle of applications running on a Windows Phone, from when the application is launched until it is terminated.
The execution model is designed to provide end users with a fast, responsive experience at all times. To achieve this, Windows Phone allows only one application to run at a time. This eliminates the possibility of users getting their device into a state where multiple applications are running in the background, causing the application in the foreground to become sluggish.