a. Teething. Teething is painful obviously, and therefore it's natural for baby to wake up wanting to be comforted.
b. Increase mobility. Max is moving around when he sleeps. One time we found his feet stuck between the rails of the crib; another time we found his head got stuck at the corner of the bed. Off course he would then cries out for mommy and daddy to help.
c. Separation anxiety. Max sleeps in his own crib, so waking up and not finding us around would cause him to scream: "MOMMY...:" well ok it's more like "WAaaaaaaa".