A visual: the right edge of the knife character actually looks a bit like a back SCRATCHer that I have, and the entire character looks to me like it could be a person with a back pack on. Perhaps the backpack doesn't fit right, so it SCRATCHes.
Picture the iconic angry dwarf on an unpleasant backpacking trip. The presence of the dwarf reminds me this is 4th tone, as he grumbles, "HUA4 (why) do people think backpacking is fun? If this thing SCRATCHes me one more time I'm outta here!"
Angry dwarf - SCRATCHy pack - HUA4 (why!)