Historically cavalry were generally the elite and they knew it. Cavalry would often make reckless charges to prove their; braver, worth, honor, valor and to gain wealth. Few cavalry untis were immune to this. So far I have only had this problem with units such as Companions, I believe Cataphracts and other non-nobles obey much better. Try that out. Yes they have some problems getting dragged into fights but the non-noble cavalry are less inclined to get dragged in and I cannot remember the last time I had that happen to me.