Perhaps a small release from what we call boredom?
It's clever really, deciding to alternate from the left and right sides, left going up, and right going down.
boredom at it's finest. when you are so bored that you have tried many of the possible keyboard combinations. this particular combination is top to bottom on the left side, bottom to top on the right side, alternating each side, starting from the far right.
When one reaches such an extraordinary level of boredom they decide it's a wonderful idea to type from an alternating pattern of down to up on the left side to down to up on the right side.