Unusual crawl and index activity from Yahoo

Indexes and ranks homepage and non-existant URL's, but no deep crawl.

[webzilla]

Yahoo crawls one of my domains with several unusual appended URL's. Examples:

[[mydomain].com...]
[[mydomain].com...]
[[mydomain].com...]
[[mydomain].com...]

And it also indexes those URL's.

They also index the home page with it's proper title tag, etc, and even ranks it very nicely for relevant terms, but at the same time Y never crawls any of the subdirectories on the site. This has been going on for about a year and the only thing they index is the home page and the oddball URL's.

Does anybody know what that is all about?