Found this bug and the reason seems a bit hard to find:
If you open Cesium Sandcastle on an iPhone and start resizing the canvas using the widget handle, eventually browser reaches almost 2GB of memory allocation and crashes.
Why this is relevant: Rotating the phone portrait / landscape orientations make all Cesium fullscreen apps crash very fast even with only showing the basemap globe.
I made a sandcastle for reproducing this:
It took 3600 iterations (runtime maybe ~30 seconds) on my iPhone to crash, and the result looks this way: