I realized that it was happening after I switched from "creative" to "survival." It would happen when there were no other apps running (I know how to close background apps), with wifi and internet turned off, and after completely restarting the device...leaving it powered down for a few hours without being plugged in. It has stopped crashing since I switched back to "creative" mode. I have 2 other limited worlds, but I haven't been playing them for a long while. I haven't had any issues with any other apps on my iPad, so it isn't a systemic issue with my device.
The world I've been in is set to "infinite."