Sorry for the late reply.
I finally know the cause. Here's a picture of what I meant by the effect of the loose Y.
The reason it came loose was because the belt clamp does not squeeze the belts tight enough and they are able to slip pass the teeth when the force is great enough through vibration.
I fixed it by gluing the belt together and inserting a piece of scrap ABS to tighten the space. I think it will work.