What triggers a multiple hit attack?

According to https://gamefaqs.gamespot.com/ps2/459841-final-fantasy-xii/answers/247302-how-can-i-do-a-combo-attack, this is a combo attack, which is randomly triggered.

http://finalfantasy.wikia.com/wiki/Hit_(term)#Final_Fantasy_XII explains it in more detail. Essentially, almost every melee weapon has a chance to trigger a combo attack, with different weapons having a different chance. The weapons a Bushi is proficient with by default happen to have a much higher chance of triggering combo attacks than most other weapons. the frequency can be increased by wearing Genji gloves. Finally, if the character triggering the combo has lower HP, each individual combo will trigger more extra attacks (up to 12 attacks are possible below 6.25% HP).