To curse
(qll) does not merely refer to the use of condescending language, but refers to a serious breach of familial duty. The verb, means to make light of something.
The prohibition refers to treating parents as if they are of little importance, treating them lightly. This is a serious attempt by some people to discredit or even destroy their parents.
9 For anyone who curses his father or his mother shall surely be put to death; he has cursed his father or his mother; his blood is upon him.