The reasons why Amazon's zip code changes automatically may include the following aspects:

1. Address update: When users modify their account address or delivery address on the Amazon platform, the Amazon system may automatically update the zip code to ensure accurate delivery of goods.

2. Geographical changes: Sometimes the government or postal department will adjust the zip code system, which may add, delete or merge certain zip code areas. If these changes affect the zip code of a certain area, the Amazon system will update the zip code accordingly.

3. Database upgrade: Amazon's database system may be regularly upgraded based on the latest zip code data. These upgrades may be to correct previous zip code errors, fix non-existent zip codes, or solve other data-related problems.