Library Constantly Updating Frustration

Does anybody know why my Aperture library would "Update" itself each and every time I close out of the library? Here's an example of what's going on. Last night I was using the library and eventualy shut it down. It then went through the Update process. I let it run over night since it's huge, 500,000 plus images. I open it again this morning, do nothing on any of the photos, decide to switch to another library and when I try to switch it goes back in ot the Update process AGAIN. Even though it did the same thing last night and nothing has been changed that needs updating. With a library this large I spend countless hours not being able to use Aperture due to this Update process. Any ideas out there?