In Parshat Vayera 18:20-21, Hashem tells Avraham that the outcry of the people of Sodom and Ammorah have reached Him. He will therefore descend to these places to see if the people have acted in a way that would prompt this outcry. What exactly is this “outcry,” and why does Hashem decide to go now to look at the cities when the people of these places have always been known to do evil as is seen in 13:13?
An answer may be learned from the story of Noach and the flood. In 6:11, the Torah says that the land became “corrupt” and filled with “injustice.” Rashi explains that the meaning of “corrupt” is the transgression of idolatry and sexual immorality, while “injustice” implies theft. Two Pesukim later in 6:13, Hashem says to Noach that the land has filled with “injustice” (i.e. theft) and therefore, the people will be destroyed. Rashi comments that this shows that the direct reason for the destruction is theft - not idolatry or promiscuity. Why was theft a bigger reason than idolatry? The reason could be that Hashem acts with mercy towards people until they commit sins that result in oppression of other innocent people.
The Yalkut Shimoni supports this in Shemot 22:22. This Pasuk says that if a person oppresses another, and the other cries out to Hashem, the He will surely listen to this cry and punish the oppressor. The Yalkut Shimoni comments that since it says when someone cries out Hashem will hear; one may think this to mean that Hashem will only listen to one who cries out. Rather, the Torah says that Hashem will “surely listen” to tell us that Hashem will always hear his cries, but He will be swift to punish if the oppressed cries out.
Therefore, in our Pasuk, we see that the nature of this “outcry” is the cry of the oppressed, and this also explains why Hashem decides to act now. It is because the cries of the people have reached Him, and He acts swiftly to punish in such cases. He did not punish them before this because he was giving the evildoers of Sodom and Ammorah time to repent and return from their bad.