Yep - the relative value on each iteration is the culprit here. There are a few ways to solve it, but I think the easiest is just to set the lines back to 0 after they fade out. Add this to the end of the timeline in the clearData() function and you'll be good to go. You also won't need relative y values on the tweens in that function with this change. Just y:20 will work perfectly.
tl.set("#para-long-line, #para-short-line", { y: 0});
Happy tweening.