Planing is usually due to using cheap, poorly made broadheads, or the wrong spine arrow, or crooked arrows (and not all carbons are straight, nor do they remain straight - contrary to that 'carbons are either straight or broken' urban legend), or insufficient fletching for the size broadhead being shot, or broadheads that aren't mounted straight and concentric with the arrow, or shooting a bow that isn't properly tuned, or shakey shooting form, or a combination of any or all those points.

To top it off, the faster the arrow is flying, the more any of those problems you might have are magnified.