Decline of Maurya empire started after the death of Emperor Ashoka in 232 BC. Seven kings succeeded Ashoka including Dasharatha, Samprati, Shalishuka, Devavarman, Satadhanvan and Brihadratha. The rulers after Ashoka ruled for almost 50 years from 232 BC to 187 BC.
The major factors that led to decline of Mauryan Empire were,
Policy of Ashoka
In his later life Ashoka became a Buddhist and gave up war. He decreased military expenditure and started spending money on public welfare. This kind of expenditure weakened the administrative setup of Maurya.
Askoka was followed by very weak rulers who could not stop foreign invasion from Central Asia. They lack the ability to crush various internal dispute. As a result many local kings declared independence.
Division of Empire
Under the rule of Ashoka, Maurya Empire was highly centralized and king was having ultimate authority. Soon after the death of Ashoka, Maurya empire was divided into two major parts. The Eastern part was ruled by Kunala and his son Samprati and the Western part was ruled by Dasratha and his successor till Brihadratha Maurya.
After Ashoka, many small kingdoms within the Maurya Empire revolted. The Andhras (Satavahanas) in the South were the first to start revolting. They were followed by Cholas , Pandya and Cheras. Various kingdoms in present day Afghanistan and eastern Persia also revolted.
Rulers like Demetrius of Indo-Greek who were nomads of Central Asia found it the best opportunity to attack India, central administartation within the nation was weak. They were highly successful in their plans and conquered deep inside India till the Indo-Gangetic Plains.
Military Coup by Pushyamitra Sunga
Last King of Maurya empire was Brihadratha Maurya. He was killed by his Army General Pushyamitra Sunga in 187 BC.
Pushyamitra Sunga established Sunga Dynasty.