The guards who had been posted by Pilate and the Jewish leaders may not have expected very much to happen. After all, the time that their leaders had recalled about the message of Jesus was that he would rise three days after his death. Of course, their concern was not that he would rise but that his disciples would try and move his body and pretend that he had risen. I doubt if the guards thought that the disciples would be much of a problem.
What would they have thought when the earthquake occurred? This was the second earthquake since Jesus died, and they must have noticed the unusual frequency of the earthquakes. Whether it surprised them or not, the arrival of the angel that accompanied the earthquake shocked them.
The guards saw a powerful individual whom they could not handle. Indeed they saw a creature that the entire Roman army could not have harmed in any way. Before they passed out, they observed the appearance and the actions of the angel. He was so bright in appearance, and he had no difficulty removing the stone from the entrance. Maybe they wondered if he had come to help the disciples of Jesus.
The posture of the angel seemed so threatening to the guards. He sat on the stone and shone in his splendour. What would he do next? The guards did not have the ability to inspect the tomb to see if the body of Jesus was in it. Instead they passed out and did not hear the wonderful message that the angel had for the women who had come to see the tomb.
Because they sided with the wrong power, the guards did not discover that Jesus had risen, that heaven was delighted, and that the followers of Jesus could now have real hope.