Of his wife, the narrator says,

One day she accompanied me, upon some household errand, into the cellar of the old building which our poverty compelled us to inhabit.

Thus, we do not know exactly why the speaker and his wife go down into their basement, but we know that it has to do with a household errand of some sort. After the fire destroyed their first home, they were compelled to move into an older building—perhaps an apartment building—with a cellar. Perhaps they went down to retrieve some household object they had stored in the basement; or perhaps they can food and the narrator was going down to retrieve some of it. There are any number of reasons that they might have gone down into the cellar, but it sounds as though it was perfectly mundane.

It is in this cellar where the narrator murders and then disposes of his wife. He decides to hide her body within the wall, as he's heard monks of the Middle Ages used to do. The walls were "loosely constructed" and the plaster had never dried completely, so he is easily able to dislodge the bricks and then repair the mess with new plaster.

The narrator does not give details of why he and his wife went into the cellar. He simply says-

 One day she accompanied me, upon some household errand, into the cellar of the old building which our poverty compelled us to inhabit.

It is evident that the wife trusted her husband, despite his unreasonable anger and strange behaviours.

 The moodiness of my usual temper increased to hatred of all things and of all mankind;

 He is humbled by her devotion and humanity, to him and to the cat he perceives as evil-

 my uncomplaining wife, alas! was the most usual and the most patient of sufferers.

It is her humanity and goodness in trying to prevent her husband from taking his axe to the cat he is tormented by night and day. His wife intervenes as he raises the axe to the cat, and she pays most cruelly for her action-

 Goaded  by the interference, into a rage more than demoniacal,  I withdrew my arm from her grasp and buried the axe in her brain.