This is often referred to as a 'position protest.' Your baby has reached a developmental milestone where they realize that being held horizontally is the precursor to being put down in a crib. Because their world has become so interesting, they are protesting the end of 'awake time' and trying to stay upright to see what is happening around them. Try an 'upright reset' by pausing the rocking, holding them upright until they are calm, and then trying a different, more vertical soothing position.