Comment by m463

sunlight?

Plastics degrade like this in sunlight due to UV.

If you put plastic storage bins outside, in a short time picking them up will usually cause them to shatter in your hands.

As a sort of cautionary tale - PEX (plastic) plumbing is quite interesting. It is almost like the fiber-optica cable of plumbing. It is really easy to install without welding and can be snaked through walls easily. However, it comes in lightproof packaging and is not to be left exposed to sunlight - otherwise it must be discarded. Holding back 100psi of water requires strong bonds.

Note that this UV exposure might have happened before you installed it. It could have been sitting in the sunlight at a distributor or depot or as a second-hand cable. It could also have been in the sunlight in your house before you pulled it.