Answer: Hybrid roses are grafted onto vigorous rootstocks. Sometimes canes grow from below the graft and are unnoticed until they overtake the hybrid canes. Other times the desired canes are accidentally pruned out because no attention was made to their point of origin. Or, sometimes winter weather will kill back the hybrid canes, and the rootstock will send out canes from below the graft, to keep the roots alive. So technically, hybrids don't revert to the rootstock, but canes from the rootstock can overtake the hybrid grafted onto it. The secret to preventing this from happening is to recognize the graft and keep any canes developing below the graft pruned off so they don't take over.
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