The cause was a hand grenade brought onto the plane by a Japanese gangster of the Yamaguchi-gumi.

[1][2][3][4][5][6][7][8][excessive citations] At around 8PM of October 26, 1986, Thai Airways International Flight 620 (Airbus A300-600, Registration HS-TAE) was cruising above Tosa Bay off the coast of Kochi Prefecture, carrying 14 crews and 233 passengers, when the rear of the aircraft suddenly suffered an explosion, resulting in rapid decompression and damage to the rear pressure bulkhead, severing two of the three hydraulic pipes.

As a result, the aircraft veered off course for about 100 kilometers and into restricted air space of the Japan Air Self Defense Force, as well as going into a dutch roll at one point, but the aircraft managed to make an emergency landing at Osaka Itami Airport at 8:40 PM.

[11] Initially, it was believed that, like Japan Air Lines Flight 123 a year before, some sort of a mechanical problem was to blame for the incident.

[2][3][5][10][14] The yakuza who brought in the hand grenade was not arrested until after he was discharged from the hospital, because the man had suffered severe burns all over his body as oil from a broken hydraulic pipe poured over him during the incident.