its just how GG works. every time a target lock is hit gives them turn meter (2% each hit 4% for grievous) and every special gives b1 turn meter (15%) which allows him to get more debuffs out. and every defuff B2 applies gives out ever more turn meter (5% each).... plus grievous AoE removes turn meter making it worse.
also bugs against him is not the best since Grievous is a AoE heavy team
edit: so if b2 applied 6 buff immunities and hit all target locks possible (at 16:53) thats about 42-48% turnmeter for everyone and 54-58% turn meter for grievous (6 debuffs, 8 target locks hit [b1 assist)
edit 2: i forgot dark side enemies also lose 5% turn meter when damaged by an attack when they have target lock... so that also doesnt help