What could be causing these cracks in the drywall?
It looks to me like the cracks are appearing on the board joins. 

Some possible causes there are:


Something went wrong with the drywall tape, or there was no tape used at all? You may want to scratch around and try to find some of the tape? I think i can actually see a bubble seam on the wall, indicating that your tape is pushing out. I would say they didn't pre-fill the join seams when applying tape -- basically a poor taping and preparation job. 




Other - Less likely Faults:


The boards may not be secure enough (ie, enough lateral supports, and my be vibrating too much (wind or knocks) 
Structural Fault - do they all follow some sort of pattern? the house may be settling and structurally changing
Excess Moisture: easy to check (if the area around the crack is damp)