At the beginning of Chapter 8, Scout notes that Mr. Avery said it was written on the Rosetta Stone that when children disobey their parents, the seasons change. When the children go to Miss Maudie's house to transfer as much snow as they can from her yard to theirs, Mr. Avery yells at them, "Hasn’t snowed in Maycomb since Appomattox. It’s bad children like you makes the seasons change.” They built the snowman in Mr. Avery's image because he accused them of being bad children (which resulted in the snow, according to his theory). Mr. Avery was trying to be a good-natured/grumpy old man.
Atticus marveled that the children were able to scrounge up enough snow to make a snowman at all. But once he recognized the resemblance to Mr. Avery, he told Jem to change it. Later that night, Mr. Avery risked injury and maybe his life fighting the fire at Miss Maudie's house.