It may be an issue with connecting to Game Center or our servers. Tiny Tower does a couple callbacks to our servers for things like making backups and checking for gifts/missions. Go into your device's Game Center Settings and log out and back into your Game Center account. Also, check your internet connection. Tiny Tower doesn't require an internet connection but if you are connected to a weak signal and the app tries to use it unsuccessfully this can cause a crash. To rule this out, try disabling the wifi on your device temporarily and see if the issue continues.
If there is a specific action that seems to cause the crashing/freezing please let us know.