The sight of something so effing cute, such as a beagle puppy playing, or any cute puppy, or baby dressed in a onesie that makes them look like a bear or some other fuzzy cute animal. Take an already cute baby, add the onesie multiplier effect and it is too much for most passionate, feeling people to take. Once the cute-rage has occurred, this energy must be dissipated by punching something, typically a large pillow which provides enough resistance for the blows to feel satisfying or hugging the kid/dog so hard their eyes pop out; as this is not an option, you hit the pillow and run around swearing about the cuteness. Cute-rage CANNOT and does not occur with inanimate objects. The DSM only defines cute-rage to occur after seeing a living thing that is off-the-charts cute, or doing something similarly cute.
Holy fawking shit, did you see that four-year-old dressed up as a lion? He had a tail, the hood with ears a la Max from Where the Wild Things Are, whiskers painted on his chubby little cheeks! When he trick-or-treated my house I almost punched a hole in the door from the cute-rage; I was so overtaken by his off-the-charts cuteness! He even growled at me with this little kid voice before the "trick-or-treat".