Write SQL query to solve given problem: Tell me the reason for Lavelle Vandervort's encounter on 11/20/2013?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT T2.REASONDESCRIPTION FROM patients AS T1 INNER JOIN encounters AS T2 ON T1.patient = T2.PATIENT WHERE T2.DATE = '2013-11-20' AND T1.first = 'Lavelle' AND T1.last = 'Vandervort'