What is it about Android that allows an application to "hard-crash" a device? I have a never-rooted Nexus 4 running latest stock (android 4.4.4). It has been "hard-crashing" lately. By "hard-crash" I mean the screen goes black and either the device restarts itself (I see the same splash screen that is displayed during boot), or I have to hold the power button for a long time to get it to start (longer than for a normal boot).

After evidence pointing to it being caused by Google's own "Google Now Launcher" app (even after a factory reset), I began to wonder what it is about an apk file installed from the Play store on a non-rooted device that could ever cause the phone to "hard-crash". I would think that the sandbox in which apps run would prevent any app from being able 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.