Most often this happens because your system ran out of memory. When the memory is running low, the iPhone will kill the currently running App so there will always be memory to receive calls. So for apps which use a lot of memory (such as games), if you are experiencing this, make sure you don't have more than one page open in Safari, stop it from loading a page before hitting the Home button, and if that doesn't work try turning the phone off and then on again (not just locking it, but holding the lock button until it gives you the option to turn off the phone) before running the app. Also check to see if there are any updates for that app, which might make it use less memory.
