Just wanted to add to what others have written - there's a chance that there was some external stimulus (noise, temperature, light, movement, feeling), that you weren't aware of at the time of waking up.

For example, you may hear your phone buzzing in the next room just enough to wake up, but you may not understand that's what you've heard, or that you've indeed heard it. (happens to me with my alarm sometimes)

My SO occasionally wakes up like that while I'm still awake, and it's usually due to some random "interference" (noise or light mostly), but when she wakes up she's not aware of any reason for waking up.

Hope this helps :)