When user had any active recurring orders and cancels his/her account (or Administrator does that), then these recurring orders will keep charging. Usually recurring orders are used to buy user's membership in the specific user group (e.g. "Premium Members" and such), but once user is deleted I not even sure to what user all these memberships will go to.
OrdersEventHandler via hook to
u:OnAfterItemDelete we need to set
IsRecurringBilling to 0 and
NextChargeOn to NULL for all deleted user orders.