The answer (based on my experience) is: moisture.
Those strong rare-earth magnets used on that model are composed from materials very prone to oxidizing (rusting). To prevent that from occurring, the magnets are coated (plated?) with a shiny metal or some other material. That seals the moisture out.
If that coating is not perfectly hermetic, some moisture can get in and start the oxidation process. That causes the coating to bulge and eventually crack and fall off.
The photo above demonstrates this. I bought these inexpensive magnet cubes on eBay several years ago. They are stored in my basement workshop where it gets humid during the summer. Over time they started to deteriorate to how they look know. One of them still looks to be ok. The coating on that one must be perfectly tight.
Even with the shiny coating gone, they are still functional, but "fuzzy", and the fuzz they shed is magnetic too. To see if I can "defuzz" one I rolled it on a piece of masking tape (the adhesive side). Most of the fuzz sticks to the tape. Once the magnet looks clean, I paint it with few coats of clear nail polish. That seems to seal it.
I guess you can try the same for your magnets. Take some maskign tape and remove the oxidation and remining shiny layer, then coat it with some lacquer. Don't use water-based paints.
