The Autofill Extension in Chrome is definitely the problem. Visiting the same pages in Safari does not generate this behavior.
It seems to be related to the fact that ZIP is also called POSTAL code which is remarkably similar to POST which happens to be the title of a new posting. This field that I'm currently typing in is called "post_content", for example.
The Autofill Extension lets you turn off that rule entirely, or exclude it by page/domain. I just added my WP site to the domain exclusion and the frustration went away. If you go to Preferences > Extensions > Autofill > Options, you'll see the choices there.
Now if only I could get over my writer's block :)