Like KonoKazuma said Slow Reflex debuff from his chase BUT there is 1 strange thing about his chase and that is sometimes Shisui needs to hit 2 chases to start his standard chain and sometimes only one which doesn't make any sense. [that 60% chance applies to everybody else but I think that it's 100% for Shisui, NEVER had a fight vs Shisui who did 2 chases and didn't start his attack chain and I fought by now hundreds of them]

Normally you hit enemy first and then apply debuff so how in the world is Shisui getting extra standard attacks from just 1 chase is enigma since his global release. Probably bugged but left on purpose to milk some $$$