口 indexes 唬 in the mouth section and points to the act of voicing: scaring or bluffing is something done through speech - shouting, growling, talking big. Pairs with 吓, 喊, 叫 in the same family of mouth-driven action verbs.
虎 gives the sound hǔ exactly. It also drips semantic flavour - the bluff or threat carries a tiger's growl. You see 虎 in 虎口, 老虎; here it both names the sound and colours the meaning.
