Santander approved application, then blocked account

I got approved for a Santander credit card last week and, once I received my card in the mail, I applied for a current account as well, for which I got approved immediately. Following that, I deposited £1000. I also requested a balance transfer, which was something I asked for in the application process but the limit they gave was lower than my ask so I requested one based on 95% of the approved credit limit.

I set the payment method for some online services I use to the new credit card and last night I tried to make a payment, which was declined.

This morning I tried to login to the app and was told to contact the Helpdesk. They ended up redirecting me to call 01908 343925, who I now understand to be the Application Fraud team. They told me to go in branch with passport, pay slip, bank statements. I now have to wait 48 hours until "they'll be in touch".

Have I been flagged for something or is it just a common occurrence to be approved for a card and account and then have id verification done. Only thing that I can think of is that some of the online subscriptions I use are in USD, so those might have blocked $1 or so each time. Thoughts?