This is just a wild guess, but as hibernate stores the information in your memory, not your HDD(that is what sleep does) it may, after a while, get to a point where it goes "oy, i need more power to keep this" so turns on.
Slightly on, slightly off topic, I would just shut the system down and not use hibernate, in so much as disable it to free hard drive space. Whatever your memory size is, so if you have 4GB of memory for instance, it will use 4GB of empty space even when NOT hibernating. I disabled mine yesterday and saved myself 8 GB of space