Pages that become Posts

I have just noticed that some static Pages on my site have become Posts. That is, they no longer display when called with wp_list_pages (and they always did), and now when I look in the Manage section, the Pages tab reports no Pages exits. I can see the Pages now listed amongst the Posts on the Manage Posts tab.

Any ideas as to why this is happening? Is it perhaps related to the frequent apparently automatic disabling of all my plugins?

I recently upgraded to 2.3.3 after being on 2.1 for a while to see if these problems might stop occurring.