check and see which page links to these pages. maybe your website's domain had a previous owner who had a webpage, and there are some inbound links pointing to that old website. this is something you can't control, if thats the case you should do a redirect on these pages to your start page. Do this with your .htaccess
file:
ErrorDocument 404 /index.html
some things to check that may produce malformed urls and is under your control:
- your paging code in search results and/or categories of products/services/content
- your sitemap
i also had a similar experience, in one of my websites i had this scheme:
example.com?category=1
example.com?category=2
example.com?category=3
and in webmaster tools i was getting random strings:
example.com?category=xxcCzxvsd
in my analytics nobody (except googlebot) ever visited example.com?category=xxcCzxvsd
. I couldn't find the source of this, so there is a strong chance the problem is in google's side.