Why are more and more of my pages going supplemental?

No onsite changes and more links!

[marvin]

I've been reading the posts here about supplemental pages and I have a good idea as to what causes urls to be put in to the supplemental index.

However, I don't understand why lots of my pages have been moved from the main index in to the supplemental index recently. About 2 months ago around 300 of my site's 440 pages were in the supplemental index. This has been slowly growing and now about 360 pages are in the supplemental index (with a corresponding reduction to the number of pages in the main index).

I find this strange as I have made only trivial changes to the pages on my site and I have received several new links over this period.

At this rate i'm getting concerned that my whole site will go supplemental before Christmas! I would welcome any suggestions as to why this is happening and how I can remedy the situation.

Regards

Marvin

[SEOPTI]

This effect is consistent with all of my sites.