MY site disappears. Why?

[oven]

My site used to be included by Yahoo, with more than 100 pages. In these few months, Yahoo removed most of my pages and only included a few useless links such as old press release and etc..

If I re-submit my homepage, it will get indexed again after 1 week, but after several days, it will be removed again!!

What goes wrong? To my understanding, we don't commit in spammy techniques such as hidden text, links, keyword stuffing, and etc...

Please help.