Most commonly, the application crashes because the mobile device is switching between Edge and 3G networks.
Information from Apple for iPhone and iPod touch users:
Applications on iPhone or iPod touch may close unexpectedly because the device has begun to run low on application memory. This doesn't mean there's too much data stored on the device; it simply means that the device is running too many memory-intensive tasks.
For tips on avoiding application crashes, please visit http://support.apple.com/kb/TS1295