for my kids night wakings like you describe are associated with bad dreams. for some reason, they don't wake agitated or upset, but really resist going back to bed. it was finally my youngest that told me. i asked (in my exhaustion) "what's so bad about sleep! mommy wants to sleep" and he asked me if i saw the "Big t-rex in my dream". then i figured out that he was actually trying to avoid sleep and was much better able to to pull it off than i.
once i knew, i was able to coach him through the nightmares.
Happy Mommy to one amazing girl (6y) and one sweet boy (2y), and wife to DH since 7/03 : :