Adrian was watching the detention center from a nearby ridge expecting rebels to try and break people out. To prevent prison radar from detecting him and his men they flew in using SHIELD wingsuits. Then the alarms went off and he gave the order for the assault unit to move in. After hacking the prison security feeds he found Steve in the server room and ordered his men to kill the power to that part of the prison and to detain any unregistered metahumans they found, with explicit orders for nonlethal detainment and not to engage Captain America; that was his job.
Sentinel stuck a kicker charge to the door and another to the adjacent wall for later use and blew the door to flood the room with smoke. In walked The Sentinel clad in red, white and blue with the shield in the same colors. "Captain America by the authority of the United States government I am ordering you to surrender." He was ready for a fight, Steve had come too far to surrender and he knew it but Adrian had his orders and severe consequences if he did not obey them.