reCAPTCHA is a security service provided by Google. On pixiv, it's used primarily to prevent automated bot attacks and unauthorized access.
It analyzes the user's behavior and environment to assess the likelihood of suspicious activity.
While Google does not disclose the specific criteria for making these calls, users deemed suspicious may be prompted to verify they are not robots by selecting the correct images in a test.
If you're having trouble with reCAPTCHA authentication, please refer to this Help section.