The link below lists some of the reasons - improper liquid in globe, stored in hot attics etc.

But it also gives tips on how to restore them. Hopefully you can fix yours too. Only one of mine has yellowed and it's probably because I had it in the attic.

If you google "why do snow globes turn yellow" you'll find some other sites too. We used to have a fellow in my town who would restore them for a low price but he's retired now.

Here is a link that might be useful: sno-globes