Taking SCARY out of used bikes, one bike at a time.
When the cable housing is too short for the brakes, they can bind and cause you to crash. In this case the rear brake housing was 3″ too short so when you turn the handlebars it would bind and THAT WILL LOCK UP YOUR REAR BRAKE!
When you turn your handlebars you should see a clean bend and clearance.
Double check yours, especially if you have changed out your stem!