Promotion methods that pay users or offer rewards specifically for installing, rating, or engaging with an app — install farms, pay-per-install networks, incentivized download campaigns — are prohibited because they artificially inflate an app's visibility and metrics.
Usually flagged after a spike in install volume from a narrow set of traffic sources that Google's fraud detection associates with known incentivized-install networks.
"We detected unusual install patterns associated with incentivized or fraudulent installs for your app, in violation of the App Promotion policy."