When a dog is aggressive towards its owner when being beaten, there may be some of the following reasons:

Self-protection Instinct: Dogs may feel threatened or in danger, and aggression is their natural reaction to try to protect themselves.

Pain and Discomfort: Spanking can cause physical pain in dogs, which can make them irritable and aggressive.

Emotional reaction: Being spanked may cause negative emotions in dogs, such as fear, anxiety or anger, causing them to be aggressive towards their owners.