It was not just random that they got kidnap. The spy took Yoon who could not fight hostage to force the group to go peacefully. If it was not due to Yoon then they won't be captured so easily.
Here is the raw link site: http://wasabisyrup.com/archives/q7zBZm6HwKA
The only reason they could force Yona and the dragons to go with them was because they used Yoon who could not fight as a hostage. If Yoon was not with them then they will not be kidnapped so easy. I don't mean to say...
