If I'm reading the syslog right it says the shutdown was from you pressing the power key. I think the default settings are that a long press just turns it off, are you sure you weren't holding the powerkey down before it turned off? Although it seems this occasion was caused by you pressing the power button, another cause can be if the contacts on the battery aren't tight enough it can lose contact with the ones on the N900, if this is a problem you can bend the contacts on the battery closer together using something like a needle.