What is it about Android that allows an application to "hard-crash" a device?
Like on any OS, although the app code itself is isolated in its own process container, it's possible for the app to trigger problems in the OS itself or in the hardware. For example, if the crash you're seeing is caused by a hardware problem such as a damaged heatsink, then any heavy workload will make the hardware fail, and there's not much the OS can do to stop that. The least reliable part of most operating systems is the drivers, so if there's a bug in (say) the GPU driver, that could make the kernel hang regardless of which process caused the situation that triggered the bug.