Both rear lights cracked

[snapdragon]

I have noticed some moisture in my rear facelift lights, the ones on the boot lid.
It is only in the reversing light band. Today I looked at them more closely and found both lenses cracked horizontally across the reversing section.
How the hell did that happen? Seemed to have happened over winter.
I will have to remove them and see if I can seal them without it looking terrible.
Anyone seen this before.