Retargeting typically attracts higher CPMs than regular display advertising. This means if a bad actor is able to generate a fake engagement on retargeting campaigns, they can steal a higher payout.
To do this, bad actors either generate fake clicks so when a user organically reengages they get the credit, or they use programmed bots to visit and engage with a website in a way that resembles the behaviour of a real and interested user. When the bots abandon carts or navigate away without converting, they appear to be high-value users and enter the websites’ retargeting lists. This fake audience of bots is then retargeted with advertising, generating revenue for the traffic source despite no legitimate advertising engagement.
Source: TrafficGuard Glossary