When the hunchbacked old lady was sent to a detention center for being an illegal immigrant, she realized that it would be where she met her death.
If you have comprehension, then you are probably going to realize something about something else. Comprehension leads to realization, after all.
You realized something so amazing that you couldn't believe it. That comprehension, wow! The realization was so big that you sat (さと) down in disbelief. If you didn't you would have fallen on the ground.