Many North Korean defectors were actually border guards. In 2012 a guard shot his superiors to flee over the border. Defectors who aren't guards are also a concern such as Shin Dong-hyuk, the only person known to have escaped the highest security interment camps.

Here is some more information on defections to South Korea

I can't say for certain that North Korean guards are told that their top priority is stopping defectors, but I wouldn't be surprised. We already have a historical precedent, the Berlin Wall. The Berlin Wall was constructed primarily for stopping defectors from East Berlin from escaping after people simply ran across or in one famous incident used a makeshift armored car to break through the checkpoints

With so many people attempting to escape North Korea every year, and with communist dictatorships having a long history of defectors, I would not be surprised if the North Korean guards were more concerned with escapees than invaders.

I couldn't find a definite answer, but some unsourced comments indicate that the worry isn't about guards defecting, but tour group members. There was an incident in 1984 where a Soviet ran across the line and sparked a firefight when a large group of KPA crossed into the Southern side pursuing him. Supposedly the third guard who faces North was added for this reason, but I can't verify it.

Highway barriers of various types have been around for some time, for instance these on the NK side.