Here are some key reasons why 536 AD is considered one of the worst years in history:

Extreme weather events - 536 AD saw extreme weather worldwide that led to crop failures, famine and societal unrest. A mysterious fog enveloped much of Europe and Asia for 18 months, causing summer temperatures to drop dramatically. Crops failed in Asia, the Middle East, North Africa and parts of Europe due to the cold weather.
Global famine - The extreme weather led to widespread crop failure and famine around the world. Procopius, a scholar in Constantinople, wrote that during this time "a most dread portent took place ... the sun gave forth its light without brightness." Mass starvation was reported in China and Ireland.
Plague outbreak - The Plague of Justinian, one of the first recorded pandemics, is believed to have originated in central Asia and spread to the Mediterranean in 541 AD, killing tens of millions of people. Some scholars believe the origins of this plague date back to 536 AD.
Societal collapse - The climate changes and ensuing famine, wars and plague in the mid-500s AD contributed to the decline and collapse of several civilizations. Researchers have linked this period to the declines of Teotihuacan in Mexico, the Sassanid Empire in Persia, the Gupta Empire in India and the Late Antique Little Ice Age.
Volcanic eruption(s) - A volcanic eruption, possibly in Iceland or Central America, spewed ash into the atmosphere that caused the global climatic changes. Ice cores and tree rings from around the world register a major volcanic event in 536 AD.
So in summary, 536 AD saw a convergence of climatic and environmental disasters, along with disease outbreaks, that led to famine, societal unrest and the fall of empires around the world. This combination of factors make it one of the worst years in recorded human history.