This can happen if you have Apps in the background using all the memory. 3D games are particularly memory-hungry. Try quitting other Apps:
(before iOS 7) Tap home, double-tap home, tap and hold an App in the bottom row, tap the (-) button for those you want to quit.
(iOS 7) Double-tap home then slide the App upwards to quit it.
If that doesn’t work, please send your crash logs, looking for files that start with Vogel CMI_:
Happily, this problem happens much less often on newer devices and on iOS 6 and 7.
Posted in: Bugs fixed in 1.1.8 to 1.1.11