All remove orphan for Nhibernate

So, I have a parent object called order, and a set of child objects called orderLineItems, with Cascade set to All-Delete-orphan in nHibernate. Here is my code.

using( var session = _sessionManager.GetSession())
        using (var transaction = session.BeginTransaction())
        {

            var order = _repository.GetOrderEagerlyByOrderId(session, fromDb.Id);
            var now = DateTime.Now;
            const string user = "GNB\\Username";
            var future = now.AddYears(1);
            var taxType = new TaxType(0, "Code", "AlternateNameE", "AlternateNameF", "NameE", "NameF", "DescriptionE", "DescriptionF", 13, now, future, user, now, user, now);
            var _serviceCatRep = new ServiceCatalogueRepository();

            var serviceCatalogueItem = _serviceCatRep.GetServiceCatalogueItemByCode(session, "VR-PASS");
            var orderLineItem1 = new OrderLineItem(0, null, "DescriptionE", "DescriptionF", 1, 10, null, null, 5, false, serviceCatalogueItem, null, user, now, user, future);

            order.OrderLineItems.Clear();

            order.OrderLineItems = order.OrderLineItems == null ? new List<IOrderLineItem> { orderLineItem1 } : new List<IOrderLineItem>(order.OrderLineItems) { orderLineItem1 };

            _repository.SaveOrUpdate(session, order);
            transaction.Commit();
        }

Error transaction.Commit();with error:

NHibernate.HibernateException: collection with cascade = "all-delete-orphan" no longer referenced the instance of the owner object

Also, what are the differences between all and all-remove-orphans with an example.