3 Major things make ABS plastic I think they are ABS, I'd have to double check) go brittle over time:

- Sunlight. We all have seen white machines go yellow, but translucent plastics denature in sunlight too in the same way, but it's not nearly as evident. When it gets *really* bad you can start to get discolouring and loss of translucency but by then it's long gone.

- Heat. Plenty of that in an iMac.

- Age. Over time and exposure to an oxygen atmosphere the surface begins to oxidise which can lead to it going brittle, especially in combination with the other two factors.