Builders form the walls by attaching drywall to metal or wooden beams, or studs," spaced evenly along the walls. In between, though, there's nothing behind the drywall to prevent it from moving due to the house settling or items bumping up against it. Drywall tears from movement occur more frequently in areas where you've already repaired the drywall since the initial crack is still present.
Any type of adhesive you've placed on the walls, such as the glue for wallpaper or tape to hang a calendar, can cause tearing as it comes off. In most cases, you'll know immediately that there's been a tear. However, you may have also created a small, unnoticeable tear when removing a piece of adhesive that has now gotten large enough to see.
If your walls are getting wet from leaks, steam or water outside, it can weaken the drywall material, which makes tears more likely. This often occurs in areas around windows, in bathrooms, along the ceiling or any area with water pipes behind the drywall.
Since drywall is a fairly thin building material, it is particularly susceptible to bumps and scratches from outside items. For example, you might notice tearing in areas that have furniture touching the wall or in high-traffic areas of the home, where people carry things that bump into the wall. In particular, the wall behind a door where the door knob touches when the door is open is likely to have damage.