As Kenneth says, the author is writing creatively, and having fun with the sound of the words. He/she is using language to suggest the sound and movement of chopping wood, in particular the repetition of the axe blows. One interpretation is that the author has invented a new verb just for fun - 'chop,chop,chop' . And, quite grammatically, the past tense of the verb 'chop,chop,chop' is ''chop,chop,chopped'.

The author is amusing him/herself with the sound of the words. "chop" is ungrammatical in this context.

Here are some other examples of the same kind of usage: "And then there came from the staircase a tremendous knock-knock-knocking. Some one had dropped her dumbbells."--Katherine Mansfield "a gust of rifle-bullets swept over and into the parapet; a Maxim rap-rap-rapped and its bullets spat hailing along the parapet above their heads." "he sat there chip, chip, chipping, down in the dusky mine"--R. M. Ballantyne, "Deep Down: A Tale of the Cornish Mines" "in the silence the two startled people could hear the rhythmic sound of the water as it drip-drip-dripped on to the floor." It's not quite the same but I can't resist quoting this one: Hear the sledges with the bells, Silver bells! What a world of merriment their melody foretells! How they tinkle, tinkle, tinkle, In the icy air of night! While the stars, that oversprinkle All the heavens, seem to twinkle With a crystalline delight; Keeping time, time, time, In a sort of Runic rhyme, To the tintinnabulation that so musically wells From the bells, bells, bells, bells, Bells, bells, bells— From the jingling and the tinkling of the bells."--Edgar Allan Poe, "The Bells"