Scout says "...the memory of Atticus calmly folding his newspaper and pushing back his hat became Atticus standing in the middle of an empty waiting street, pushing up his glasses."

I think that the two incidents being compare are the night the lynch mob tried to get Tom Robinson and Atticus shooting the dog Tim Johnson. Note that both names (Tim and Tom) reflect each other. Scout's memory of the lynch mob seems to merge with her memory of the shooting of the dog. There are parallels to both images. There is Atticus butting a wounded dog out of his misery as if Tom Robinson might warrant the same charity. The dog also reflects the angry mob unable to see reason behind their hate. In both cases, Atticus is seen as the calm example of humanity trying to make the best of a bad situation.