Same thing happens to me, OP. Sometimes I put it into sleep mode and it turns itself off. It's really annoying how it freaks out when you turn it back on, all like, "OHSHIT YOU TURNED ME OFF DURING SLEEP MODE, SOMETHING MIGHT'VE BEEN FUCKED UP, GOTTA DOUBLE CHECK EVERYTHING!!! ...Oh, everything's okay, carry on."
In fact, I just took a glance at my PS4, and it seems to have done it again.
I don't understand how the Wii U's sleep mode is more reliable than the PS4's. I mean it doesn't have suspend/resume, sure, but at least it doesn't turn off or freak the fuck out if it gets unplugged for whatever reason.