One of the actions for defining custom alerts are spikes and plummets. In the second window of configuring a custom alert (Select Metric To Use for Alert 2/4), 

you can pick these actions and navigate to the Explanation tab to view their definitions.

For example with downloads, the spike and plummet alerts trigger when the number of downloads for an app significantly increases or decreases compared to its downloads over the previous 15 days. Sensor Tower uses a combination of statistical analyses including standard deviation, mean, and median, to determine when these spikes and plummets trigger.