That's not "Falling asleep and not waking up" - there is a known issue in some devices where they go into sleep mode and won't recover.
This is just the phone rebooting at random from a lock up. It sounds like that a static charge built up in one of the circuits. This caused it to shut off. And, you had to pull the battery to allow that charge to dissipate before it would power back on.
This is common in solid state electronics. This static charge is the reason we support people tell users (As annoying as it may seem) to power off devices for several seconds and back on. Wireless routers, Cable/DSL modems, Digital cable tv boxes, Cell phones, Computers.... All are subject to this.
What causes it?
For the most part, it's just simple physics. In the most basic of terminology, Electronic circuits are just metal with electrons moving around in them. This movement of electrons causes friction. This is what causes the heat you feel in any electronic device. It also causes a static charge build up (Think rubbing a balloon on your head and sticking it to the wall).
99% of the time, this static just dissipates on its on. Sometimes, there is just too much for it to dissipate. Maybe the air around the device is too charged.
We have a technical term for when this happens to any electronic device that causes you to have to pull the power source for a few seconds and let it reset.... "
$h!t Happens."
LoL
If it's a "once in a blue moon" event, it's nothing to worry about.
If it starts happening on a regular basis, it could be something wrong with the phone, or it could be something in the environment where you are using the phone causing it. Are you using the phone around anything with large electric motors, anything that gives off high EMF, in an area with really dry air that is statically charged (as in you keep shocking your cat on the nose when you touch it!)????
yup 300 hp dc motors thru the plant