It happens quite frequently because of the way ad networks work. Ads are not vetted or regulated in any meaningful way and often ad networks will serve up ads

It happens quite frequently because of the way ad networks work. Ads are not vetted or regulated in any meaningful way and often ad networks will serve up ads from other networks so a content provider has no way of knowing the true source of the ads they're serving. There's been a violation of the trust between content providers and consumers. Consumers have found ad blockers as a means to protect themselves and it's up to providers to now come up with a safer arrangement.