Man explodes cell phone on Line 5 train... What happened? (South Korea)

A man in his 60s was arrested by police after he started a fire on a moving subway in South Korea by hitting a mobile phone with a shovel. Seoul Yangcheon Police Station arrested a man in his 60s on charges of arson and assault on business on the 27th of last month.
According to police, at around 10:33 a.m. on the same day, they received a report that smoke was coming from a train passing through Mokdong Station.
At the time, A was on a moving train and hit his mobile phone with a shovel he was carrying, causing the phone to explode and smoke to escape.
A passenger on the same train stopped A, and when the train arrived at Mokdong Station, he threw the broken mobile phone over the train.
The fire was extinguished after the fire was stopped. The stationmaster and other staff members quickly managed the safety of the scene, and no injuries were reported. A, who was arrested at the scene, was handed over to the police at Mokdong Station, and the Mokdong Stationmaster
The remains of the mobile phone were recovered and handed over to the police. A police official said, "We plan to investigate the specific circumstances of the incident, including whether A was drinking or had a mental illness."