This is most likely the merchant choose to enable 3-D Secure for large transactions so they're not liable for frauds. 3-D Secure flow is separate from the norm

This is most likely the merchant choose to enable 3-D Secure for large transactions so they're not liable for frauds. 3-D Secure flow is separate from the normal authorization flow and require the merchant (or payment gateway) to set it up. The merchant (or the gateway) is required to build or purchase an MPI from an approved vendors to make it work.