Ivan IV Vasilyevich (Russian: Ivan Vasil'evich, tr. Ivan Vasilyevich; 25 August 1530 - 28 March [O.S. 18 March] 1584), commonly known as Ivan the Terrible or Ivan the Fearsome (Russian:  Ivan Groznyi  , Ivan Grozny; a better translation into modern English

Conditions under Oprichnina were worsened by the 1570 epidemics of plague that killed 10,000 people in Novgorod. In Moscow it killed 600-1,000 daily. During the grim conditions of the epidemics, famine and ongoing Livonian War, Ivan grew suspicious that noblemen of the wealthy city of Novgorod were planning to defect, placing the city itself into the control of the Grand Duchy of Lithuania. In 1570 Ivan ordered the Oprichniki to raid the city. The Oprichniki burned and pillaged Novgorod and the surrounding villages, and the city was never to regain its former prominence.  Casualty figures vary greatly in different sources. The First Pskov Chronicle estimates the number of victims at 60,000. According to the Third Novgorod Chronicle, the massacre lasted for five weeks. The massacre of Novgorod consisted of men, women and children were tied to sleighs, which were then run into the freezing waters of the Volkhov River, which Ivan ordered on the basis of unproved accusations of treason and tortured its inhabitants and killed thousands in a pogrom there, the archbishop was also hunted to death. Almost every day 500 or 600 people were killed or drowned. Yet the official death toll named 1,500 of Novgorod's big people (nobility) and mentioned only about the same number of smaller people. Many modern researchers estimate the number of victims to range from 2,000-3,000 (after the famine and epidemics of the 1560s the population of Novgorod most likely did not exceed 10,000-20,000). Many survivors were deported elsewhere.  Oprichnina did not live long after the sack of Novgorod. During the 1571-72 Russo-Crimean war, oprichniks failed to prove themselves worthy against a regular army. In 1572, Ivan abolished the Oprichnina and disbanded his oprichniks.

Using a quote from the above article, answer the following question: Why did they sack Novgorod?
Ivan grew suspicious that noblemen of the wealthy city of Novgorod were planning to defect,