You need to declare that the "OrderLines" should be lazy loaded. You could use the FetchPlans API or declare the association to be loaded lazily explicitly in the configuration.
If you using code-only mapping you can define the load behavior:
orderLineConfiguration.HasAssociation(x => x.Order).WithLoadBehavior(Telerik.OpenAccess.LoadBehavior.Lazy).WithOpposite(c => c.OrderLines);
The OrderLines will now be lazy loaded. Please refer to the documentation here and here.